I took the liberty to merge a fix removing a trailing space that I had introduced in the earlier suggestion...

Thanks @oscargus and @QuLogic for your reviews. I took this opportunity to take another look at the rules for docstrings in matplotlib (this referring to numpydoc] and it says

When a parameter can only assume one of a fixed set of values, those values can be listed in braces, with the default appearing first:

order : {'C', 'F', 'A'}
   Description of `order`.

so I reordered the options accordingling and removed the explict mentioning of the default value in the docstring.

I further noticed that loc in Cell has a default value of None

which is then set to right in the code:

so I took the liberty to change the default directly to 'right'. Formally speaking, this is an API change but in practice this will only have any effect if someone would explitely use Cell(... loc=None). The only place where Cell() is used in the matplotlib code is in add_cell and non of the calls of add_cell pass None for loc. However, if you feel this is too unsafe a change I will be happy to reverse it.

Could you add an api change note if it isn't too much trouble?

What is the API change? The default value in the signature changed, but the run time behavior should not have changed.

The default value in the signature changed

Isn't that usually something we flag as an API change even if the behavior doesn't change? Just in case someone was relying on it?
ETA: @StefRe explains why #26696 (comment)

Formally speaking, this is an API change but in practice this will only have any effect if someone would explitely use Cell(... loc=None).

As I said before, I'm happy with any of the three options:

1. change the default loc paramter in Cell to 'right' but don't add an API change note (this is what this PR currently is)
2. change the default loc paramter in Cell to 'right' and add an API change note
3. leave the default loc paramter in Cell unchanged

I tried to find an answer to the question about API changes in the documentation and found this:

• If in doubt, decisions about API changes are finally made by the API consistency lead developer

which leads to the question who the API consistency lead developer is.

who the API consistency lead developer is.

@timhoffm - that info lives at https://matplotlib.org/governance/people.html#deputy-project-leads

No API change note is a bit too adventurous. If we wanted to be very defensive, the removal of None support would have to go through a deprecation cycle. We may dare to remove it immediately, but we at least have to document this in an API change note.

@story645 I added the API change note. The Sphinx FileNotFoundError: [Errno 2] No such file or directory: '/tmp/pip-build-env-cidrmif2/normal/bin/ninja' seems to be unrelated to this PR.

You'll need to rebase to fix CI.

Thanks for going the extra steps of making the API note!