Add documentation; fix MappingType defaults

Adds a bunch of docstrings.

MappingType should be overridden. The get_indexes and
get_mapping_type_name methods were returning default values that
didn't make any sense. Better to raise NotImplementedError.
Overhaul query docs

* changes some of the formatting so that we get interlinking for
  free
* add a bunch of things to the API section
* remove "verbose content" that duplicates content in the docstrings
  and in some cases was out-of-date or just plain wrong