gh-94787: [doc] Add to argparse doc an example of a mutually-exclusive group nested in an argument group

[scottamain]

It's not obvious that a group can be added to a group, and this is important when trying to organize your help output.

(New main branch PR originally from PR #94788)

• Issue: add_mutually_exclusive_group() suggests that you cannot add help groups #94787

[bedevere-bot]

Most changes to Python require a NEWS entry.

Please add it using the blurb_it web app or the blurb command-line tool.

[scottamain]

Friendly nudge. @slateny

[slateny]

Actually, per #66246 some forms of nesting may not actually be supported, and there's also a deprecation note underneath the changes mentioning that nesting with mutually exclusive group is deprecated.

[scottamain]

Okay, but this change is about adding a mutex group to a regular arg group (to get help title/description), not to another mutex group, which I believe is what all those other conversations are about. So I've just updated the PR with some copy changes to clarify that. Is it okay in this manner?

[slateny]

Since Irit said it's supported, the clarifications lgtm :)

[slateny]

lgtm thanks, will tag Irit per the reply in the issue

[iritkatriel]

Thank you, @scottamain.

[scottamain]

My pleasure, thank you for your prompt attention and contribution, @iritkatriel and @slateny