ClickToFocus (which enables click-to-focus, as opposed to
focus-follows-mouse) is undocumented in the man page.

The story seems to be:
- Claude implemented it way back, but kept it undocumented because
  he didn't use it himself:
http://tigerdyr.wheel.dk/ctwm-archive/0654.html
http://tigerdyr.wheel.dk/ctwm-archive/0867.html
- it was "correctly implemented" in 3.6 at the same time as
  SloppyFocus was added:
http://ctwm.free.lp.se/CHANGES.html

Upstream haven't documented it so far, and I can't be bothered to
find the right contact there (sorry!).

BR,
Jörgen