rustc_error_codes: improve docs for E0642

Author: Ezrashaw

compiler/rustc_error_codes/src/error_codes/E0642.md

@@ -1,4 +1,8 @@
-Trait methods currently cannot take patterns as arguments.
+Trait methods cannot take patterns as arguments.
+
+Traits cannot control the way in which implementations take arguments, only the
+types of those arguments. A pattern here has no effect, because the
+implementation can bind its arguments however it would like.
 
 Erroneous code example:
 
@@ -16,3 +20,9 @@ trait Foo {
     fn foo(x_and_y: (i32, i32)); // ok!
 }
 ```
+
+And you can use patterns in its implementations:
+
+impl Foo for Bar {
+    fn foo((x, y): (i32, i32)) { /* ... */ } // ok!
+}