Reference wildcard patterns from underscore expr

These appear in nearly identical locations in source code and have subtly different semantics, so cross link them.

Author: riking

src/expressions/underscore-expr.md

@@ -8,6 +8,8 @@ Underscore expressions, denoted with the symbol `_`, are used to signify a
 placeholder in a destructuring assignment. They may only appear in the left-hand
 side of an assignment.
 
+Note that this is distinct from the [wildcard pattern](../patterns.html#wildcard-pattern).
+
 An example of an `_` expression:
 
 ```rust