DOC: clarify `limits` parameter in `winsorize` docstring.
STY: PEP8 the `scipy.stats.mstats.winsorize` function.
DOC: add short explanation to `scipy.stats.mstats.winsorize` doctring.
Merge pull request #3309 from andreas-h/gh2971

Doc enhancements to scipy.stats.mstats.winsorize