DOC: `pd.Categorical` has undocumented `fastpath`
### 

- [X] I have checked that the issue still exists on the latest versions of the docs on `master` [here](https://pandas.pydata.org/docs/dev/)


### Location of the documentation

https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Categorical.html#pandas.Categorical

### Documentation problem

Argument `fastpath` is not documented.

### Suggested fix for documentation

All arguments to the function should be documented. `fastpath` is not currently documented even though it is part of the function signature.