Proposed documentation enhancement

The current documentation for the option 'mode' of mne.sensitivity_maps refers to a "manual" (presumbably the C-version of MNE), but there is no link (see below). I think the different choices should be explained in the documention of MNE-Python, without reference to a manual.

mode: str
The type of sensitivity map computed. See manual. Should be 'free', 'fixed', 'ratio', 'radiality', 'angle', 'remaining', or 'dampening' corresponding to the argument --map 1, 2, 3, 4, 5, 6, 7 of the command mne_sensitivity_map.