Improve button action mapping documentation

Source: http://sourceforge.net/p/linuxwacom/patches/88/
Signed-off-by: Jason Gerecke <killertofu@gmail.com>
Remove vestiges of "displaytoggle" action mapping

This was removed from the driver in 2010 (910dc637). Stop claiming to
support it in xsetwacom(1), etc.

Source: http://sourceforge.net/p/linuxwacom/bugs/236/
Signed-off-by: Jason Gerecke <killertofu@gmail.com>