DOC: Add offsets methods to the API reference
The methods of classes in `pandas.tseries.offsets` are not in the API reference. Not sure if there is a reason for it, or it just needs to be implemented.

We have the classes added to:
https://pandas-docs.github.io/pandas-docs-travis/api.html#date-offsets

But most of the attributes and methods are not in `api.rst`, so their pages are not generated. See for example:
https://pandas-docs.github.io/pandas-docs-travis/generated/pandas.tseries.offsets.MonthEnd.html#pandas.tseries.offsets.MonthEnd

Can they simply be added? Or is there a reason not to, or it's more complex than it looks like?

CC: @jbrockmendel @mroeschke 