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).

@jorisvandenbossche I would say this closes #5813 the issue of wether certain datetime index methods should exist on Series as well is addressed by another issue 

(actually it's pretty easy to fix just need a common mixin for series/DatetimeIndex)

the issue though is that these should only apply if the Series has an appropriate dtype (so need some error checking)
 @jorisvandenbossche I take that back

#5813 is about Timestamp - so fine to leave open

same approach is needed though - a mixin should work
 Very good @jorisvandenbossche.  I didn't see that it was Timestamp either.  It's not clear where that method comes from (it has to be on the _Timestamp class, but where is that?).  Nevertheless, thank you for adding to the DatetimeIndex docs!
 The method comes directly from datetime.datetime if I am not mistaken (as Timestamp subclasses datetime). E.g. if you type `pd.Timestamp.weekday?`, you will see the correct docstring from datetime. 
So reagarding issue #5813, the thing left could be to a) add some explanation to one of the tutorials, and/or b) add Timestamp to the api.rst (but this causes problems, see #5218).
 @jorisvandenbossche looks fine...merge?
 @jreback For me it's OK to merge. I suppose the way I add the docstring is rather straightforward.
