DOC: Add DateOffset and subclasses to api.rst
xref https://github.com/pandas-dev/pandas/pull/22288#issuecomment-414757610

`DateOffset` and its subclasses are only described in `timeseries.rst` and currently do not exist in `api.rst`. Since this represents a good portion of pandas timeseries functionality, we should probably be highlighting them in `api.rst`.
