Add more std::io documentation.

This round: io::Result and the free functions.

src/libstd/io/error.rs

@@ -17,11 +17,37 @@ use option::Option::{self, Some, None};
 use result;
 use sys;
 
-/// A type for results generated by I/O related functions where the `Err` type
-/// is hard-wired to `io::Error`.
+/// A specialized [`Result`][result] type for I/O operations.
+///
+/// [result]: ../result/enum.Result.html
+///
+/// This type is broadly used across `std::io` for any operation which may
+/// produce an error.
 ///
 /// This typedef is generally used to avoid writing out `io::Error` directly and
-/// is otherwise a direct mapping to `std::result::Result`.
+/// is otherwise a direct mapping to `Result`.
+///
+/// While usual Rust style is to import types directly, aliases of `Result`
+/// often are not, to make it easier to distinguish between them. `Result` is
+/// generally assumed to be `std::result::Result`, and so users of this alias
+/// will generally use `io::Result` instead of shadowing the prelude's import
+/// of `std::result::Result`.
+///
+/// # Examples
+///
+/// A convenience function that bubbles an `io::Result` to its caller:
+///
+/// ```
+/// use std::io;
+///
+/// fn get_string() -> io::Result<String> {
+///     let mut buffer = String::new();
+///
+///     try!(io::stdin().read_line(&mut buffer));
+///
+///     Ok(buffer)
+/// }
+/// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub type Result<T> = result::Result<T, Error>;