DEPR: Changing default of str.extract(expand=False) to str.extract(expand=True)
…o str.extract(expand=True) (#6581)

- [X] related to #6581
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

I changed the section in the wantsnew, and updated the text and the issue number, but I left the default parameter.

In the signature, `expand` had a default value of `None`, but in practice [1] it was False, as it was also said in the docstring [2]. I'll change the message if you disagree, but I think it makes more sense to say that by default was `False` before.

1. https://github.com/pandas-dev/pandas/blob/master/pandas/core/strings.py#L687
2. https://github.com/pandas-dev/pandas/blob/master/pandas/core/strings.py#L613 Thank you for the comments @jreback I understood it now. If you want to take a look, I think it should be much better, and with the examples now. Let me know if any other change is needed. Thanks for the comments @TomAugspurger. I made the changes. In text.rst, there was no example with expand unspecified or None, so the text was already correct. I added a quick note about the default value in the warning.

Please let me know if that looks good, or you've got a better idea. All the comments from the reviews were addressed, right? Just double checking that the PR doesn't need anything from my side. Thanks for the info @jorisvandenbossche, didn't know about the `.. ipython::` directive.

It should be all right now. @datapythonista Thanks!