DOC: fixup categorical news
DOC: update cut/qcut docstrings

* add some clarification about the useage of bins/labels and that they
  end up as categories.
* Update the examples to produce the current output with the new
  categoricals.
Fixup 10min
Doc: Add Categorical to R comparison

Also fix all omited `pd.`
Merge pull request #8413 from JanSchulz/CategoricalFixups3

Categorical doc fixups