Documented copy, ref, move in patterns.

Closes #3337

doc/rust.md

@@ -2206,7 +2206,12 @@ fn main() {
 }
 ~~~~
 
-A pattern that's just a variable binding,
+Patterns that bind variables default to binding to a copy of the matched value. This can be made
+explicit using the ```copy``` keyword, changed to bind to a borrowed pointer by using the ```ref```
+keyword, or to a mutable borrowed pointer using ```ref mut```, or the value can be moved into
+the new binding using ```move```.
+
+A pattern that's just an identifier,
 like `Nil` in the previous answer,
 could either refer to an enum variant that's in scope,
 or bind a new variable.