XWorld: cleanup

 - Comment keymaps.h
 - Tie XWORLD_DEBUG into ROCKBOX_HAS_LOGF to ease debugging
 - Fix up the manual a little bit

Change-Id: I12cfb58001199036cd67dbaa27f164e6790a199d
Reviewed-on: http://gerrit.rockbox.org/1084
Reviewed-by: Michael Giacomelli <giac2000@hotmail.com>