Rollup merge of rust-lang#73621 - poliorcetics:mut-keyword, r=steveklabnik

Document the mut keyword

Partial fix for rust-lang#34601.

Documentation for the `mut` keyword. I think it's okay for it to be quite short, this is not the book not the reference, but if you find something is missing, do not hesitate to tell me.

Author: Manishearth

src/libstd/keyword_docs.rs

@@ -983,11 +983,61 @@ mod move_keyword {}
 
 #[doc(keyword = "mut")]
 //
-/// A mutable binding, reference, or pointer.
+/// A mutable variable, reference, or pointer.
 ///
-/// The documentation for this keyword is [not yet complete]. Pull requests welcome!
+/// `mut` can be used in several situations. The first is mutable variables,
+/// which can be used anywhere you can bind a value to a variable name. Some
+/// examples:
 ///
-/// [not yet complete]: https://github.com/rust-lang/rust/issues/34601
+/// ```rust
+/// // A mutable variable in the parameter list of a function.
+/// fn foo(mut x: u8, y: u8) -> u8 {
+///     x += y;
+///     x
+/// }
+///
+/// // Modifying a mutable variable.
+/// # #[allow(unused_assignments)]
+/// let mut a = 5;
+/// a = 6;
+///
+/// assert_eq!(foo(3, 4), 7);
+/// assert_eq!(a, 6);
+/// ```
+///
+/// The second is mutable references. They can be created from `mut` variables
+/// and must be unique: no other variables can have a mutable reference, nor a
+/// shared reference.
+///
+/// ```rust
+/// // Taking a mutable reference.
+/// fn push_two(v: &mut Vec<u8>) {
+///     v.push(2);
+/// }
+///
+/// // A mutable reference cannot be taken to a non-mutable variable.
+/// let mut v = vec![0, 1];
+/// // Passing a mutable reference.
+/// push_two(&mut v);
+///
+/// assert_eq!(v, vec![0, 1, 2]);
+/// ```
+///
+/// ```rust,compile_fail,E0502
+/// let mut v = vec![0, 1];
+/// let mut_ref_v = &mut v;
+/// ##[allow(unused)]
+/// let ref_v = &v;
+/// mut_ref_v.push(2);
+/// ```
+///
+/// Mutable raw pointers work much like mutable references, with the added
+/// possibility of not pointing to a valid object. The syntax is `*mut Type`.
+///
+/// More information on mutable references and pointers can be found in```
+/// [Reference].
+///
+/// [Reference]: ../reference/types/pointer.html#mutable-references-mut
 mod mut_keyword {}
 
 #[doc(keyword = "pub")]