BUG : relax validation on hist label

Closes #3869
MNT : simplified handling of wrong number of labels

instead of padding length of labels by hand, use `zip_longest`
DOC : numpydoc-ified ax.hist docstring

 - [ .. | ..| ] -> {.., ..}
 - moved deault values to description body per the 'Paramters' section
   of
   https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt#sections
 - corrected some default values
 - added `None` to parameters where allowed
Merge pull request #3952 from tacaswell/hist_labels

Relax hist label validation