updated hist documentation
pandas.DataFrame.hist()/pandas.Series.hist() currently say that the `bins` keyword argument must be an integer, with a default value of 10. Since these methods are built off matplotlib, these actually seem to be more flexible (allowing sequences as well as integers). I don't know if there was a reason for this discrepancy, but if not, I've adjusted the pandas docstrings to reflect the matplotlib documentation.

[Existing pandas documentation](https://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.hist.html)
[Existing matplotlib documentation](https://matplotlib.org/api/_as_gen/matplotlib.axes.Axes.hist.html)

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
