Update clone reference to include closures

Author: conradludgate

src/special-types-and-traits.md

@@ -80,6 +80,7 @@ types:
 
 * Types with a built-in `Copy` implementation (see above)
 * [Tuples] of `Clone` types
+* [Closures] that only capture values of `Clone` types
 
 ## `Send`