Datetime units not listed in pd.to_timedelta docstring
#### Problem description

I wanted to do a month timedelta but upon checking the [pd.to_timedelta docstring](https://github.com/pandas-dev/pandas/blob/v0.23.4/pandas/core/tools/timedeltas.py#L26) I gave up on this as an option because there was no 'M' option. I then tried many other round-the-houses methods until someone answered [this question](https://stackoverflow.com/questions/52904433/how-to-add-subtract-an-array-of-varying-month-timedeltas-in-pandas/52904490#52904490) on stackoverflow.

This is not updated in the most [recent docs](https://pandas-docs.github.io/pandas-docs-travis/generated/pandas.to_timedelta.html?highlight=to_timedelta#pandas.to_timedelta) but would be good to include, I believe.

#### Expected Output

To see full list of (or reference to) all units that can be used.

#### Output of ``pd.show_versions()``
23.4