DOC: update api.rst for Categorical accessors / methods
we have a ``Categorical`` section, but this mainly describes ``.cat``. Maybe split this out into a separate section to document the actual ``Categorical`` methods / accessors (even though it is *slightly* duplicative.

http://pandas-docs.github.io/pandas-docs-travis/api.html#categorical

also need to add other public methods (that are not available on ``.cat``). This is actually a big list, things like:

``isnull``, ``unique``, ``map``, ``value_counts``, etc.

These are all Series like methods / accessors.