Various consistency and phrasing fixes in std::collections' docs

Various consistency and phrasing fixes in std::collections' docs

* Changed btree_map's and hash_map's Entry (etc.) docs to be consistent
* Changed VecDeque's type and module summary sentences to be consistent
  with each other as well as with other summary sentences in the module
* Changed HashMap's and HashSet's summary sentences to be less redundantly
  phrased and also more consistant with the other summary sentences in the
  module
* Also, added an example to Bound