ROIPlotter.plot_sources needs a docstring and better parameter handling

[HealthyPear]

I think this is supposed to be the method to use if I want to see how the ROI looks around a direction in the sky which is not the center of the analysis ROI.

The signature reads,

plot_sources(skydir, labels, plot_kwargs, text_kwargs, **kwargs)

where labels is a list of strings and a required positional argument even if there is a show_labels boolean (not clear from where it should be injected) and it is not clear from the source code (at least to me) which labels they are (all sources in the catalog, some sources they users should select a priori, etc...)

also plot_kwargs, text_kwargs seems to be required, but they are kwargs, so they should have defaults and not be required.

I think this method (as many others) would benefit from a clear docstring which explains what it does and what is not required with they defaults (or pointing to the where the default values are defined).