Allow passing a mask to NanOps
- [x] closes #22764
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] pytest --doctest-modules pandas/core/nanops.py
- [TO DO] whatsnew entry

Added Docs String and Examples to NanOps

Hello @alimcmaster1! Thanks for updating the PR.

 - There are no PEP8 issues in the file [`pandas/core/nanops.py`](https://github.com/alimcmaster1/pandas/blob/dcea82d371ffa107c17e1861a6df87082656a52a/pandas/core/nanops.py) !

 - There are no PEP8 issues in the file [`pandas/tests/test_nanops.py`](https://github.com/alimcmaster1/pandas/blob/dcea82d371ffa107c17e1861a6df87082656a52a/pandas/tests/test_nanops.py) !



##### Comment last updated on September 30, 2018 at 18:00 Hours UTC can you rebase on master # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22865?src=pr&el=h1) Report
> :exclamation: No coverage uploaded for pull request base (`master@08ecba8`). [Click here to learn what that means](https://docs.codecov.io/docs/error-reference#section-missing-base-commit).
> The diff coverage is `97.67%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22865/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/22865?src=pr&el=tree)

```diff
@@            Coverage Diff            @@
##             master   #22865   +/-   ##
=========================================
  Coverage          ?   92.19%           
=========================================
  Files             ?      169           
  Lines             ?    50828           
  Branches          ?        0           
=========================================
  Hits              ?    46862           
  Misses            ?     3966           
  Partials          ?        0
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <97.67%> (?)` | |
| #single | `42.36% <55.81%> (?)` | |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/22865?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/nanops.py](https://codecov.io/gh/pandas-dev/pandas/pull/22865/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbmFub3BzLnB5) | `95.2% <97.67%> (ø)` | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22865?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/22865?src=pr&el=footer). Last update [08ecba8...dacdbec](https://codecov.io/gh/pandas-dev/pandas/pull/22865?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @alimcmaster1 