Argument 'fastpath' in 'Categorical' not documented

[david-cortes]

In documentaion for pandas.Categorical:
https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Categorical.html

Argument fastpath which can be passed to the class constructor is not documented on the docstring:
https://github.com/pandas-dev/pandas/blob/master/pandas/core/arrays/categorical.py

[jreback]

see #20110

we would actually like to remove these

[timhoffm]

At least a short note "deprecated" or "to be deprecated" or "internal only" on the parameter would help so that users know they don't have to bother with this parameter.