Document bucket

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

Author: JustForFun88

src/map.rs

@@ -7658,6 +7658,7 @@ mod test_map {
     }
 
     #[test]
+    #[allow(clippy::needless_borrow)]
     fn test_extend_ref_kv_tuple() {
         use std::ops::AddAssign;
         let mut a = HashMap::new();