DOCS: fix category docs
- [x] closes #46770
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

The docs state that 

> all instances of ``CategoricalDtype`` compare equal to a ``CategoricalDtype(None, False)``,

which isn't true, e.g.:

```python
>>> import pandas as pd
>>> c1 = pd.CategoricalDtype(["a", "b"])
>>> c1 == pd.CategoricalDtype(None, False)
False
>>> c1 == pd.CategoricalDtype()
False
```

Users should just compare like they already do, likely `c1 == "category"` or `isinstance(c1, pd.CategoricalDtype)`. 

Thanks @topper-123 