Auto merge of #365 - JustForFun88:doc_bucket, r=Amanieu

Document `bucket`

Well, I tried to make documentation for the `bucket`, and tried to do it correctly. But maybe I missed something. You can freely edit, add or even throw everything out if  necessary 🙂.

Author: bors

src/map.rs

@@ -7657,6 +7657,7 @@ mod test_map {
     }
 
     #[test]
+    #[allow(clippy::needless_borrow)]
     fn test_extend_ref_kv_tuple() {
         use std::ops::AddAssign;
         let mut a = HashMap::new();