DOC: Fix documentation for DataFrame.groupby.transform
 - The doc template should no longer reference "Series" transforms explicitly.
 - Make consistent the formatting for references to function argument names.
looks fine. @rhshadrach if any comments. Thanks @vladu! Thanks, Richard. I appreciate your feedback. This was my first PR to pandas, and I was trying to do something low impact / low controversy. Will be more comprehensive going forward. 

-- Vlad

 @vladu - I wouldn't recommend changing a thing. To make sure - my complaint about the current docs for `DataFrame.transform` was very much outside the scope of this PR. This was well done.