Clarify all borrow operators in the precedence table

This removes `&` and `&mut` and instead just links to the full grammar
for what the borrow operators are. There are many other variants, like
`&&` or `&raw`, and I didn't want to write all of them down. The
keywords after the `&&` are just modifiers, and are not really separate
things to be considered anyways.

Closes #1726

Author: ehuss

src/expressions.md

@@ -81,7 +81,7 @@ Binary Operators at the same precedence level are grouped in the order given by
 | [Field expressions][expr.field] | left to right   |
 | [Function calls][expr.call], [array indexing][expr.array.index] | |
 | [`?`][expr.try]             |                     |
-| Unary [`-`][expr.negate] [`!`][expr.negate] [`*`][expr.deref] [`&`][expr.operator.borrow] [`&mut`][expr.operator.borrow]  | |
+| Unary [`-`][expr.negate] [`!`][expr.negate] [`*`][expr.deref] [borrow][expr.operator.borrow] | |
 | [`as`][expr.as]             | left to right       |
 | [`*`][expr.arith-logic] [`/`][expr.arith-logic] [`%`][expr.arith-logic] | left to right       |
 | [`+`][expr.arith-logic] [`-`][expr.arith-logic] | left to right       |