DEPR: Deprecate str.split return_type
Closes #9847, Closes #9870. Default value is `expand=False` to be compat with`return_type='series'`. May better to change the default to True in future (and show warning about it)?

CC @jreback @jorisvandenbossche @sreejata

@jreback, @mortada Thanks, updated.
 Looks good! 

Another thing I encountered while reviewing: there is no good explanation of the `n` kwarg of split in the docstring, and the default differs between `str_split`/docstring (default of None) and the signature of `split` itself (default of -1)
 @jorisvandenbossche Explanation of `n` is written as notes, thus moved it to main part. Also, modified the docstring to meet the `str.split` without changing the default values. Or may better to change defaults as None and -1 has the same meaning?

I also found current arg names are different from standard `str.split` (`sep` and `maxsplit` ) ...
 merged via https://github.com/pydata/pandas/commit/8b8984258cbede113d362c18508f85fcef4eaaf5

@sinhrks thank you!

if you have minor doc changes, pls fee free to make another PR.
