Update pkgid-spec docs.

Author: ehuss

Diff for: src/doc/src/reference/pkgid-spec.md

@@ -4,37 +4,60 @@
 
 Subcommands of Cargo frequently need to refer to a particular package within a
 dependency graph for various operations like updating, cleaning, building, etc.
-To solve this problem, Cargo supports Package ID Specifications. A specification
+To solve this problem, Cargo supports *Package ID Specifications*. A specification
 is a string which is used to uniquely refer to one package within a graph of
 packages.
 
+The specification may be fully qualified, such as
+`https://github.com/rust-lang/crates.io-index#regex:1.4.3` or it may be
+abbreviated, such as `regex`. The abbreviated form may be used as long as it
+uniquely identifies a single package in the dependency graph. If there is
+ambiguity, additional qualifiers can be added to make it unique. For example,
+if there are two versions of the `regex` package in the graph, then it can be
+qualified with a version to make it unique, such as `regex:1.4.3`.
+
 #### Specification grammar
 
 The formal grammar for a Package Id Specification is:
 
 ```notrust
-pkgid := pkgname
-       | [ proto "://" ] hostname-and-path [ "#" ( pkgname | semver ) ]
+spec := pkgname
+       | proto "://" hostname-and-path [ "#" ( pkgname | semver ) ]
 pkgname := name [ ":" semver ]
 
 proto := "http" | "git" | ...
 ```
 
 Here, brackets indicate that the contents are optional.
 
+The URL form can be used for git dependencies, or to differentiate packages
+that come from different sources such as different registries.
+
 #### Example specifications
 
-These could all be references to a package `foo` version `1.2.3` from the
-registry at `crates.io`
-
-| pkgid                        | name  | version | url                    |
-|:-----------------------------|:-----:|:-------:|:----------------------:|
-| `foo`                        | `foo` | `*`     | `*`                    |
-| `foo:1.2.3`                  | `foo` | `1.2.3` | `*`                    |
-| `crates.io/foo`              | `foo` | `*`     | `*://crates.io/foo`    |
-| `crates.io/foo#1.2.3`        | `foo` | `1.2.3` | `*://crates.io/foo`    |
-| `crates.io/bar#foo:1.2.3`    | `foo` | `1.2.3` | `*://crates.io/bar`    |
-| `https://crates.io/foo#1.2.3`| `foo` | `1.2.3` | `https://crates.io/foo` |
+The following are references to the `regex` package on `crates.io`:
+
+| Spec                                                        | Name    | Version |
+|:------------------------------------------------------------|:-------:|:-------:|
+| `regex`                                                     | `regex` | `*`     |
+| `regex:1.4.3`                                               | `regex` | `1.4.3` |
+| `https://github.com/rust-lang/crates.io-index#regex`        | `regex` | `*`     |
+| `https://github.com/rust-lang/crates.io-index#regex:1.4.3`  | `regex` | `1.4.3` |
+
+The following are some examples of specs for several different git dependencies:
+
+| Spec                                                      | Name             | Version  |
+|:----------------------------------------------------------|:----------------:|:--------:|
+| `https://github.com/rust-lang/cargo#0.52.0`               | `cargo`          | `0.52.0` |
+| `https://github.com/rust-lang/cargo#cargo-platform:0.1.1` | <nobr>`cargo-platform`</nobr> | `0.1.1`  |
+| `ssh://git@github.com/rust-lang/regex.git#regex:1.4.3`    | `regex`          | `1.4.3`  |
+
+Local packages on the filesystem can use `file://` URLs to reference them:
+
+| Spec                                   | Name  | Version |
+|:---------------------------------------|:-----:|:-------:|
+| `file:///path/to/my/project/foo`       | `foo` | `*`     |
+| `file:///path/to/my/project/foo#1.1.8` | `foo` | `1.1.8` |
 
 #### Brevity of specifications