DOC: improve the string repr of "no_default" in the signature of methods
- [x] closes https://github.com/noatamir/pyladies-berlin-sprints/issues/20

> For example in https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.dropna.html, the signature shows "how=_NoDefault.no_default". This could be improved to the shorter / less noisy "how=<no_default>", by adding a __str__ method to this object (https://github.com/pandas-dev/pandas/blob/7e5a95cda77d2665d5bec2f4154bf72f041ac36b/pandas/_libs/lib.pyx#L2671-L2678)

@phofl and @noatamir
@AliceAML I don't think this produced the expected change.

I checked by building the docs locally and downloading the docs built by the CI and in both cases I see "how=_NoDefault.no_default" at [pandas.DataFrame.dropna](https://pandas.pydata.org/docs/dev/reference/api/pandas.DataFrame.dropna.html). Here's a comparison of the dev Docs and the file from the CI:

<img width="1253" alt="comparing live docs (dev) with CI artifact" src="https://user-images.githubusercontent.com/6564007/211873030-bb8a7643-d9d1-4bf3-95db-3a80ea59f88e.png">
 My bad, I assumed that sphinx would use `str(..)` of the object to display it in the docs. If that doesn't work, not fully sure what sphinx is doing ..

Can you check if `str(pd.api.extensions.no_default)` actually gives the expected result? (because maybe it is something with the Enum that it's not working as expected) @jorisvandenbossche yeah that works for me on this branch.

![check str(pd api extensions no_default)](https://user-images.githubusercontent.com/6564007/211885169-1a77b847-8272-4e8a-880c-af388661db22.png) I'll have to take a closer look here, right now I am guessing that sphinx is doing something weird here This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. @AliceAML thanks for the PR. closed as stale but if you want to continue, please comment and will reopen.