Add warning for sharex/sharey when subplots=False in plot method

[MehdiNemri]

• closes Ignore sharex unless subplots=True #60299
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[mroeschke]

Thanks for the PR, but the issue hasn't had much discussion on a resolution so I think it may be too early to open a PR for this.

That being said, this is probably more appropriate as a documentation inclusion instead of a warning

[mroeschke]

Closing as the issue needs more discussion first