Rollup merge of rust-lang#38186 - frewsxcv:default, r=GuillaumeGomez

Add docs for last undocumented `Default` `impl`.

Add doc comment for `Default` `impl` on `DefaultHasher`.

Fixes rust-lang#36265.

Author: GuillaumeGomez

Diff for: src/libstd/collections/hash/map.rs

@@ -2117,6 +2117,10 @@ impl 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()
     }