TST: remove all docstrings from test functions for stats.morestats
TST: add tests for stats.probplot, and fix minor issues and docs.

Closes gh-677 (Statistics Review).
MAINT: clean up some more code in stats/morestats.py
MAINT: address review comments of @josef-pkt on gh-2991.

Adds an example for stats.probplot, and accepts more inputs for the dist
keyword.
Merge pull request #2991 from rgommers/review-probplot

Test and fix up stats.probplot