doc: Add FAQ about overload() and mouse events

[softmoth]

This question has been raised as an issue many times. It continues to trip up new users, especially given that the recommended configuration includes the capslock, esc overload which suffers from this problem.

[frankier]

Looks good. I think you can get this from the man page, but this would have saved me a bit of confusion. It might be also good to refer to this FAQ from the "quickstart" configuration -- since many probably like me just copypaste that and move on.