MAINT: clean up stats/mstats_extra.py a little.
DOC: fix a couple of issues in stats docstrings.

Code in double backticks should be on a single line.
``stats.theilslopes`` plot wasn't rendering, apparently text in docstring
examples cannot be at the end.
MAINT: clean up stats/mstats_basic.py a little.
Merge pull request #3949 from rgommers/statsdocs

DOC: fix a couple of issues in stats docstrings.
Merge pull request #3947 from rgommers/mstats-cleanup

MAINT: some docstring fixes and style cleanups in stats.mstats
DOC: stats: Small formatting correction to the 'chi' distribution docstring.
Merge pull request #3952 from WarrenWeckesser/doc-stats-chi

DOC: stats: Small formatting correction to the 'chi' distribution docstring