Remove experimental warning from custom offsets
Per discussion here: https://github.com/pandas-dev/pandas/pull/17554#pullrequestreview-63229522

- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

I edited the docstring descriptions slightly.  Feel free to suggest cleaner descriptions, if my edits don't suffice.  Should there be a whatsnew entry related to removing the experimental status?

