DOC: Add missing public plotting functions to the docs

[datapythonista]

Some plotting public functions are missing from the documentation.

As proposed in #28177 I think those should be moved out of pandas or deprecated, but I guess it's worth having them documented as the rest of similar functions until that happens.

[WillAyd]

Looks like the docstrings for these functions don't pass the validator - any chance you can clean those up as part of this?

[datapythonista]

Thanks for the head ups @WillAyd, didn't see it was failing.

Fixed the errors that we validate for now, I think it's worth to wait until a decision is made on #28177 before working in detail with those docstrings.

[TomAugspurger]

Thanks!