DOC: Improving docstring of pop method (#16416)
 - [ ] closes #xxxx
 - [ ] tests added / passed
 - [X] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

As discussed in gitter, it'd be nice to have more and better examples in the API documentation. I think it'd be nice to have as many examples as possible with the same DataFrame, so the users who check the documentation know it without needing to read it again.

In this PR I propose a standard DataFrame that:
 - Is simple to understand
 - Contains all the main dtypes
 - Contains NaNs
 - It can be groupped

Please feel free to propose a another one if you have a better idea, and reject this PR.

Let me know if the format of this docstring, as well as the DataFrame can be used for the rest of the method docstrings, and I'll work with them.

You can see how the html documentation looks like in the next screenshot:

![pandas_pop_doc](https://cloud.githubusercontent.com/assets/10058240/26524333/883d96e6-4327-11e7-856c-31cc3446b268.png)
