Auto merge of #328 - JustForFun88:update_hashbrown_doc_1, r=Amanieu
Auto merge of #328 - JustForFun88:update_hashbrown_doc_1, r=Amanieu

Improving documentation.

Improved hashbrown documentation. Changed some examples so that they become full-fledged tests.

rust-lang/hashbrown,rust-lang/hashbrown