Improve clarity of scalar types

Attempts a more holistic fix to #535 which unifies language around built-in scalars and serialization, shortening the intro to this section in the process.

Author: leebyron

spec/Section 3 -- Type System.md

@@ -327,18 +327,14 @@ represent additional fields a GraphQL client only accesses locally.
 ScalarTypeDefinition : Description? scalar Name Directives[Const]?
 
 Scalar types represent primitive leaf values in a GraphQL type system. GraphQL
-responses take the form of a hierarchical tree; the leaves on these trees are
+responses take the form of a hierarchical tree; the leaves of this tree are
 GraphQL scalars.
 
-All GraphQL scalars are representable as strings, though depending on the
-response format being used, there may be a more appropriate primitive for the
-given scalar type, and server should use those types when appropriate.
-
-GraphQL provides a number of built-in scalars, but type systems can add
-additional scalars with semantic meaning. For example, a GraphQL system could
-define a scalar called `Time` which, while serialized as a string, promises to
-conform to ISO-8601. When querying a field of type `Time`, you can then rely on
-the ability to parse the result with an ISO-8601 parser and use a
+GraphQL provides a number of built-in scalars (see below), but type systems can
+add additional scalars with semantic meaning. For example, a GraphQL system
+could define a scalar called `Time` which, while serialized as a string,
+promises to conform to ISO-8601. When querying a field of type `Time`, you can
+then rely on the ability to parse the result with an ISO-8601 parser and use a
 client-specific primitive for time. Another example of a potentially useful
 custom scalar is `Url`, which serializes as a string, but is guaranteed by
 the server to be a valid URL.
@@ -348,25 +344,31 @@ scalar Time
 scalar Url
 ```
 
-A server may omit any of the built-in scalars from its schema, for example if a
-schema does not refer to a floating-point number, then it may omit the
-`Float` type. However, if a schema includes a type with the name of one of the
-types described here, it must adhere to the behavior described. As an example,
-a server must not include a type called `Int` and use it to represent
-128-bit numbers, internationalization information, or anything other than what
-is defined in this document.
+**Built-in Scalars**
+
+GraphQL provides a basic set of well-defined Scalar types: {Int}, {Float},
+{String}, {Boolean}, and {ID}. A GraphQL server framework should support all of
+these types, and a GraphQL server which provides a type by these names must
+adhere to the behavior described below. As an example, a server must not include
+a type called {Int} and use it to represent 64-bit numbers, internationalization
+information, or anything other than what is defined in this document.
+
+When returning the set of types from the `__Schema` introspection type, all
+referenced built-in scalars must be included. If a built-in scalar type is not
+referenced anywhere in a schema (there is no field, argument, or input field of
+that type) then it must not be included in the schema.
 
 When representing a GraphQL schema using the type system definition language,
-the built-in scalar types should be omitted for brevity.
+all built-in scalars must be omitted for brevity.
 
-**Result Coercion**
+**Result Coercion and Serialization**
 
 A GraphQL server, when preparing a field of a given scalar type, must uphold the
 contract the scalar type describes, either by coercing the value or producing a
 field error if a value cannot be coerced or if coercion may result in data loss.
 
 A GraphQL service may decide to allow coercing different internal types to the
-expected return type. For example when coercing a field of type `Int` a boolean
+expected return type. For example when coercing a field of type {Int} a boolean
 `true` value may produce `1` or a string value `"123"` may be parsed as base-10
 `123`. However if internal type coercion cannot be reasonably performed without
 losing information, then it must raise a field error.
@@ -376,6 +378,10 @@ the precise rules of coercion are left to the implementation. The only
 requirement is that the server must yield values which adhere to the expected
 Scalar type.
 
+GraphQL scalars are serialized according to the serialization format being used.
+There may be an appropriate serialized primitive for each given scalar type, and
+the server should produce those serialized primitives when appropriate.
+
 **Input Coercion**
 
 If a GraphQL server expects a scalar type as input to an argument, coercion
@@ -394,12 +400,6 @@ input value.
 For all types below, with the exception of Non-Null, if the explicit value
 {null} is provided, then the result of input coercion is {null}.
 
-**Built-in Scalars**
-
-GraphQL provides a basic set of well-defined Scalar types. A GraphQL server
-should support all of these types, and a GraphQL server which provide a type by
-these names must adhere to the behavior described below.
-
 
 ### Int
 
@@ -409,7 +409,7 @@ that type to represent this scalar.
 
 **Result Coercion**
 
-Fields returning the type `Int` expect to encounter 32-bit integer
+Fields returning the type {Int} expect to encounter 32-bit integer
 internal values.
 
 GraphQL servers may coerce non-integer internal values to integers when
@@ -444,10 +444,10 @@ should use that type to represent this scalar.
 
 **Result Coercion**
 
-Fields returning the type `Float` expect to encounter double-precision
+Fields returning the type {Float} expect to encounter double-precision
 floating-point internal values.
 
-GraphQL servers may coerce non-floating-point internal values to `Float` when
+GraphQL servers may coerce non-floating-point internal values to {Float} when
 reasonable without losing information, otherwise they must raise a field error.
 Examples of this may include returning `1.0` for the integer number `1`, or
 `123.0` for the string `"123"`.
@@ -471,9 +471,9 @@ and that representation must be used here.
 
 **Result Coercion**
 
-Fields returning the type `String` expect to encounter UTF-8 string internal values.
+Fields returning the type {String} expect to encounter UTF-8 string internal values.
 
-GraphQL servers may coerce non-string raw values to `String` when reasonable
+GraphQL servers may coerce non-string raw values to {String} when reasonable
 without losing information, otherwise they must raise a field error. Examples of
 this may include returning the string `"true"` for a boolean true value, or the
 string `"1"` for the integer `1`.
@@ -493,9 +493,9 @@ representation of the integers `1` and `0`.
 
 **Result Coercion**
 
-Fields returning the type `Boolean` expect to encounter boolean internal values.
+Fields returning the type {Boolean} expect to encounter boolean internal values.
 
-GraphQL servers may coerce non-boolean raw values to `Boolean` when reasonable
+GraphQL servers may coerce non-boolean raw values to {Boolean} when reasonable
 without losing information, otherwise they must raise a field error. Examples of
 this may include returning `true` for non-zero numbers.
 
@@ -509,8 +509,8 @@ other input values must raise a query error indicating an incorrect type.
 
 The ID scalar type represents a unique identifier, often used to refetch an
 object or as the key for a cache. The ID type is serialized in the same way as
-a `String`; however, it is not intended to be human-readable. While it is
-often numeric, it should always serialize as a `String`.
+a {String}; however, it is not intended to be human-readable. While it is
+often numeric, it should always serialize as a {String}.
 
 **Result Coercion**
 
@@ -583,8 +583,8 @@ type Person {
 }
 ```
 
-Where `name` is a field that will yield a `String` value, and `age` is a field
-that will yield an `Int` value, and `picture` is a field that will yield a
+Where `name` is a field that will yield a {String} value, and `age` is a field
+that will yield an {Int} value, and `picture` is a field that will yield a
 `Url` value.
 
 A query of an object value must select at least one field. This selection of