Add the syntax of references to their documentation summary.

Without this change, in <https://doc.rust-lang.org/1.63.0/std/#primitives>,
`reference` is the only entry in that list which does not contain the
syntax by which the type is named in source code. With this change, it
contains them, in roughly the same way as the `pointer` entry does.

library/core/src/primitive_docs.rs

@@ -1223,7 +1223,7 @@ mod prim_usize {}
 #[doc(alias = "&")]
 #[doc(alias = "&mut")]
 //
-/// References, both shared and mutable.
+/// References, `&T` and `&mut T`.
 ///
 /// A reference represents a borrow of some owned value. You can get one by using the `&` or `&mut`
 /// operators on a value, or by using a [`ref`](../std/keyword.ref.html) or

library/std/src/primitive_docs.rs

@@ -1223,7 +1223,7 @@ mod prim_usize {}
 #[doc(alias = "&")]
 #[doc(alias = "&mut")]
 //
-/// References, both shared and mutable.
+/// References, `&T` and `&mut T`.
 ///
 /// A reference represents a borrow of some owned value. You can get one by using the `&` or `&mut`
 /// operators on a value, or by using a [`ref`](../std/keyword.ref.html) or