DOC: add way to document DatetimeIndex field attributes
+ doc for month and dayofweek
Merge pull request #5814 from jorisvandenbossche/doc-dtindex-attribute

DOC: add way to document DatetimeIndex field attributes
ENH: enhance apply() on Panel for arbitrary functions (rather than just ufuncs) (GH1148)
ENH: add property dtypes on Panel objects
ENH: allow multiple axes to be passed to axis on a Panel to process slabs