fix #30499, document behavior of return at top level (#30716)

also add some missing doc strings and improve others

(cherry picked from commit 0bb0332)

base/Enums.jl

@@ -7,6 +7,11 @@ export Enum, @enum
 
 function basetype end
 
+"""
+    Enum{T<:Integer}
+
+The abstract supertype of all enumerated types defined with [`@enum`](@ref).
+"""
 abstract type Enum{T<:Integer} end
 
 (::Type{T})(x::Enum{T2}) where {T<:Integer,T2<:Integer} = T(bitcast(T2, x))::T

base/docs/basedocs.jl

@@ -345,7 +345,9 @@ kw"function"
 """
     return
 
-`return` can be used in function bodies to exit early and return a given value, e.g.
+`return x` causes the enclosing function to exit early, passing the given value `x`
+back to its caller. `return` by itself with no value is equivalent to `return nothing`
+(see [`nothing`](@ref)).
 
 ```julia
 function compare(a, b)
@@ -371,12 +373,15 @@ function test2(xs)
     end
 end
 ```
-In the first example, the return breaks out of its enclosing function as soon as it hits
+In the first example, the return breaks out of `test1` as soon as it hits
 an even number, so `test1([5,6,7])` returns `12`.
 
 You might expect the second example to behave the same way, but in fact the `return`
 there only breaks out of the *inner* function (inside the `do` block) and gives a value
 back to `map`. `test2([5,6,7])` then returns `[5,12,7]`.
+
+When used in a top-level expression (i.e. outside any function), `return` causes
+the entire current top-level expression to terminate early.
 """
 kw"return"
 
@@ -407,7 +412,7 @@ kw"if", kw"elseif", kw"else"
 """
     for
 
-`for` loops repeatedly evaluate the body of the loop by
+`for` loops repeatedly evaluate a block of statements while
 iterating over a sequence of values.
 
 # Examples
@@ -425,8 +430,8 @@ kw"for"
 """
     while
 
-`while` loops repeatedly evaluate a conditional expression, and continues evaluating the
-body of the while loop so long as the expression remains `true`. If the condition
+`while` loops repeatedly evaluate a conditional expression, and continue evaluating the
+body of the while loop as long as the expression remains true. If the condition
 expression is false when the while loop is first reached, the body is never evaluated.
 
 # Examples
@@ -473,19 +478,23 @@ kw"end"
 """
     try/catch
 
-A `try`/`catch` statement allows for `Exception`s to be tested for. For example, a
-customized square root function can be written to automatically call either the real or
-complex square root method on demand using `Exception`s:
+A `try`/`catch` statement allows intercepting errors (exceptions) thrown
+by [`throw`](@ref) so that program execution can continue.
+For example, the following code attempts to write a file, but warns the user
+and proceeds instead of terminating execution if the file cannot be written:
 
 ```julia
-f(x) = try
-    sqrt(x)
+try
+    open("/danger", "w") do f
+        println(f, "Hello")
+    end
 catch
-    sqrt(complex(x, 0))
+    @warn "Could not write file."
 end
 ```
 
-`try`/`catch` statements also allow the `Exception` to be saved in a variable, e.g. `catch y`.
+The syntax `catch e` (where `e` is any variable) assigns the thrown
+exception object to the given variable within the `catch` block.
 
 The power of the `try`/`catch` construct lies in the ability to unwind a deeply
 nested computation immediately to a much higher level in the stack of calling functions.
@@ -561,7 +570,9 @@ kw"continue"
 """
     do
 
-Create an anonymous function. For example:
+Create an anonymous function and pass it as the first argument to
+a function call.
+For example:
 
 ```julia
 map(1:10) do x
@@ -842,7 +853,7 @@ nothing
 """
     Core.TypeofBottom
 
-The singleton type containing only the value `Union{}`.
+The singleton type containing only the value `Union{}` (which represents the empty type).
 """
 Core.TypeofBottom
 
@@ -1246,7 +1257,7 @@ Unsigned
 """
     Bool <: Integer
 
-Boolean type.
+Boolean type, containing the values `true` and `false`.
 """
 Bool
 

base/iostream.jl

@@ -4,6 +4,12 @@
 
 const sizeof_ios_t = Int(ccall(:jl_sizeof_ios_t, Cint, ()))
 
+"""
+    IOStream
+
+A buffered IO stream wrapping an OS file descriptor.
+Mostly used to represent files returned by [`open`](@ref).
+"""
 mutable struct IOStream <: IO
     handle::Ptr{Cvoid}
     ios::Array{UInt8,1}

base/strings/io.jl

@@ -440,7 +440,24 @@ function unescape_string(io, s::AbstractString)
 end
 unescape_string(s::AbstractString) = sprint(unescape_string, s, sizehint=lastindex(s))
 
+"""
+    @b_str
+
+Create an immutable byte (`UInt8`) vector using string syntax.
 
+# Examples
+```jldoctest
+julia> v = b"12\\x01\\x02"
+4-element Base.CodeUnits{UInt8,String}:
+ 0x31
+ 0x32
+ 0x01
+ 0x02
+
+julia> v[2]
+0x32
+```
+"""
 macro b_str(s)
     v = codeunits(unescape_string(s))
     QuoteNode(v)

base/sysimg.jl

@@ -186,7 +186,32 @@ include("abstractarraymath.jl")
 include("arraymath.jl")
 
 # define MIME"foo/bar" early so that we can overload 3-arg show
+"""
+    MIME
+
+A type representing a standard internet data format. "MIME" stands for
+"Multipurpose Internet Mail Extensions", since the standard was originally
+used to describe multimedia attachments to email messages.
+
+A `MIME` object can be passed as the second argument to [`show`](@ref) to
+request output in that format.
+
+# Examples
+```jldoctest
+julia> show(stdout, MIME("text/plain"), "hi")
+"hi"
+```
+"""
 struct MIME{mime} end
+
+"""
+    @MIME_str
+
+A convenience macro for writing [`MIME`](@ref) types, typically used when
+adding methods to `show`.
+For example the syntax `show(io::IO, ::MIME"text/html", x::MyType) = ...`
+could be used to define how to write an HTML representation of `MyType`.
+"""
 macro MIME_str(s)
     :(MIME{$(Expr(:quote, Symbol(s)))})
 end