Fix returns in the doc of sc.pl.violin

[yuanx749]

Close #2135.

Changed the docstring of pl.violin to be consistent with the code, and docs of other similar plotting functions.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (8986921) 72.85% compared to head (8316a69) 72.85%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2585   +/-   ##
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  Coverage   72.85%   72.85%           
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  Files         111      111           
  Lines       12383    12383           
=======================================
  Hits         9022     9022           
  Misses       3361     3361           

Files	Coverage Δ
scanpy/plotting/_anndata.py	84.97% <ø> (ø)