DOC: document optional arguments on plot submethods
As a follow up on #9321:

We should start adding optional arguments that are common between multiple plot types to the signatures for plotting sub-methods, such as `figsize` and `axis`. Currently, these are only documented on the main `.plot()` methods and we use `**kwargs` for the rest. This would be nice because we could clarify for users which optional arguments work for which types of plots -- this is not at all obvious right now.

@JanSchulz [suggests](https://github.com/pydata/pandas/pull/9321#discussion_r39086063) we could perhaps do this dynamically by updating the `__signature__` object. However, I don't know yet if this will generate the correct API documentation -- maybe if Sphinx is run using Python 3?
