Merge pull request #3158 from alexrp/incoming

Operator-assignment expressions -> Compound assignment expressions.

Author: brson

doc/rust.md

@@ -1927,13 +1927,15 @@ x <- copy y;
 
 The former is just more terse and familiar.
 
-#### Operator-assignment expressions
+#### Compound assignment expressions
 
 The `+`, `-`, `*`, `/`, `%`, `&`, `|`, `^`, `<<`, `>>`, and `>>>`
 operators may be composed with the `=` operator. The expression `lval
 OP= val` is equivalent to `lval = lval OP val`. For example, `x = x +
 1` may be written as `x += 1`.
 
+Any such expression always has the [`nil`](#primitive-types) type.
+
 #### Operator precedence
 
 The precedence of Rust binary operators is ordered as follows, going