documentation of "zscore" and "channels" Overview-mode about mne-qt-browser

[marsipu]

@sappelhoff The documentation for the different overview-modes is a bit hidden, you can find it when hovering over the mode-selection-checkbox.
Would you rather recommend something like a separate "Help"-Button next to it?

[marsipu]

This Overview-Bar might be subject to further change anyway (making channels-mode obsolete since its lack of information) as
@larsoner and @drammock discussed in #20, I opened a new issue (#27) about that.

[sappelhoff]

Okay, found it, thanks :-)

Yes, a "help" button would be better (more easily findable). An alternative to that might be to either:

1. let the hover-box appear much faster, and let it linger for a bit after the cursor moves away
2. force-open the hover-box when a user opens the "select box", that is, when they click on the dropdown menu to select either channels or zscore

[marsipu]

I fixed this in #29.
I went for your first suggestion since I think that the tool tip is a bit quicker than an extra dialog for the help which has to be clicked away then. And just hovering lets you see what you might change before you change it. Thank you for your suggestions