Add note in doc

[ruuskas]

This is related to PR 13019 in MNE-Python and closes #262.

I also tried changing the example mne_inverse_label_connectivity.py to add an example use case. However, it appears that since the figure is rendered before the plot_connectivity_circle function exits, changes made afterwards do not become visible in the figure unless fig.show() is called, which leads to showing the figure twice. Workarounds to prevent this of course exist but might clutter the example unnecessarily. Thoughts?

[drammock]

thanks @ruuskas, looks good. For this one we'll wait to merge until the upstream change is in and we've formalized what the namespace structure will be like (so that we're certain that fig.mne.colorbar is the correct way to access this)

Marking as "request changes" just as a reminder that we shouldn't merge yet.