Document when types have OS-dependent sizes

As per issue #43601, types that can change size depending on the
target operating system should say so in their documentation.

I used this template when adding doc comments:

 The size of a(n) <name> struct may vary depending on the target
 operating system, and may change between Rust releases.

For enums, I used "instance" instead of "struct".

src/libstd/io/stdio.rs

@@ -30,18 +30,27 @@ thread_local! {
 ///
 /// This handle is not synchronized or buffered in any fashion. Constructed via
 /// the `std::io::stdio::stdin_raw` function.
+///
+/// The size of a StdinRaw struct may vary depending on the target operating
+/// system, and may change between Rust releases.
 struct StdinRaw(stdio::Stdin);
 
 /// A handle to a raw instance of the standard output stream of this process.
 ///
 /// This handle is not synchronized or buffered in any fashion. Constructed via
 /// the `std::io::stdio::stdout_raw` function.
+///
+/// The size of a StdoutRaw struct may vary depending on the target operating
+/// system, and may change between Rust releases.
 struct StdoutRaw(stdio::Stdout);
 
 /// A handle to a raw instance of the standard output stream of this process.
 ///
 /// This handle is not synchronized or buffered in any fashion. Constructed via
 /// the `std::io::stdio::stderr_raw` function.
+///
+/// The size of a StderrRaw struct may vary depending on the target operating
+/// system, and may change between Rust releases.
 struct StderrRaw(stdio::Stderr);
 
 /// Constructs a new raw handle to the standard input of this process.