Clarify max_length in zstd/zlib decompression documentation

[Dreamsorcerer]

It's not that obvious from the docs today how to actually read the full output from compressed data when using the max_length parameter (zstd doesn't even mention the requirement to check .eof).

---

📚 Documentation preview 📚: https://cpython-previews--143805.org.readthedocs.build/

[picnixz]

(Sorry, I struggled to find the correct title :'))

[emmatyping]

Thanks for catching the missing docs for compression.zstd!

I think the zlib doc change isn't correct however, so please remove it.

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[Dreamsorcerer]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@emmatyping: please review the changes made to this pull request.

[emmatyping]

Sorry, I'll take a look at this this weekend.

[Dreamsorcerer]

I also opened a discussion here about improving the actual API:
https://discuss.python.org/t/zlib-decompress-should-have-an-attribute-to-tell-if-output-is-available/106898