DOC: improve DataFrame/SeriesGroupBy.apply doc string
- [x ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

The doc strings for the ``apply`` method on GroupBy objects are currently IMO difficult to "translate" into working code. This PR proposes making the doc string more practical, including:
* split the doc string up for dataframe and series GroupBy object
* have some working examples
* advice people to use other methods if possible because of speed penalty.

