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

Hello @mitchnegus! Thanks for updating the PR.



Cheers ! There are no PEP8 issues in this Pull Request. :beers: 

##### Comment last updated on January 30, 2018 at 20:49 Hours UTC # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19366?src=pr&el=h1) Report
> Merging [#19366](https://codecov.io/gh/pandas-dev/pandas/pull/19366?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/238499ab0a48a0ad4a2011e2ce1c6a02c86124eb?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19366/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19366?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #19366   +/-   ##
=======================================
  Coverage   91.62%   91.62%           
=======================================
  Files         150      150           
  Lines       48724    48724           
=======================================
  Hits        44642    44642           
  Misses       4082     4082
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.99% <ø> (ø)` | :arrow_up: |
| #single | `41.74% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19366?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/plotting/\_core.py](https://codecov.io/gh/pandas-dev/pandas/pull/19366/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb3JlLnB5) | `82.41% <ø> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19366?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19366?src=pr&el=footer). Last update [238499a...d0e44e4](https://codecov.io/gh/pandas-dev/pandas/pull/19366?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @mitchnegus do you have a chance to update based on these comments? Yes, sorry, I got busy and had forgot about this. Thanks for the bump! thanks!