DOC: add way to document DatetimeIndex field attributes

Ping @rockg. The docstring is now listed in the autosummary table in api.rst.

Related to issue #5813 (but does not closes it, this only documents the DatetimeIndex field in the api.rst, not more general the Timestamp values).

I added the attribute docstring within the `_field_accessor` function. Is this a good approach? 
You can also document attributes with a docstring line beneath the definition (see Sphinx docs: http://sphinx-doc.org/ext/autodoc.html#directive-autoattribute). The problem with this is that, for the moment, this works for the sphinx autodoc (so the generated pages), but not for the autosummary (there is an open PR for this: https://bitbucket.org/birkenfeld/sphinx/pull-request/142/make-autosummary-work-with-module-class/diff).