Add documentation for spectrogram plot args

Created by: areeda

Spectrogram plotting on https://gwpy.github.io/docs/latest/api/gwpy.spectrogram.Spectrogram.html#gwpy.spectrogram.Spectrogram

The plot function says:

plot(**kwargs)[source]

    Plot the data for this Spectrogram

It would be nice if we had a link describing those arguments, like cmap, vmin, vmax...