DOC: Include missing holiday observance rules
#### Location of the documentation

[Timeseries Holiday](https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#holidays-holiday-calendars) section in doc/source/user_guide/timeseries.rst.

#### Documentation problem

The list of available holiday observance rules is incomplete - several available observances are missing.

#### Suggested fix for documentation

The following observances should be added to the documentation because they are already available:

- next_workday
- previous_workday
- before_nearest_workday
- after_nearest_workday

Before I examined `holiday` via TAB-completion I had written my own version of weekend_to_monday; others may likewise re-invent these wheels if they're not documented.

(Updated to remove "weekend_to_monday" because it does the same thing as "next_monday")