Added doc about behavior of extend on HashMap

It was unclear what the implementation does when it encounters existing keys. This change makes it clear by documenting the trait impl.

src/libstd/collections/hash/map.rs

@@ -2362,6 +2362,8 @@ where
     }
 }
 
+/// Inserts all new key-values from the iterator and replaces values with existing
+/// keys with new values returned from the iterator.
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<K, V, S> Extend<(K, V)> for HashMap<K, V, S>
 where