Add doc comment for Default impl on DefaultHasher.

src/libstd/collections/hash/map.rs

@@ -2108,6 +2108,10 @@ pub fn new() -> DefaultHasher {
 
 #[stable(feature = "hashmap_default_hasher", since = "1.13.0")]
 impl Default for DefaultHasher {
+    /// Creates a new `DefaultHasher` using [`DefaultHasher::new`]. See
+    /// [`DefaultHasher::new`] documentation for more information.
+    ///
+    /// [`DefaultHasher::new`]: #method.new
     fn default() -> DefaultHasher {
         DefaultHasher::new()
     }