Update plot docs when backend is set.

[TomAugspurger]

As part of the backend callback, can / should we copy the docstring from the backend to the DataFrame.plot object (including methods like DataFrame.plot.scatter)?

There may be backend-specific notes or behavior. It'd be nice if those were transferred over.

cc @jsignell.

[jsignell]

Just for reference, this is where I've been working on the hvPlot entrypoint holoviz/hvplot#319

[jsignell]

I guess the question is should the docs be over-written with the docs from the backend, or should the docs be augmented with additional docs?

Also worth thinking about how this will work when #28622 is added since that will make it impossible to know which backend to show docs for. The backend set in the options.plotting.backend could just always be used.

I guess I am leaning towards not overwriting the docs, since the positional arguments will have to be the same. I do think there could be a way to overwrite the examples. And I could go through and generalize references to matplotlib.