ftrace: Append wakeup_rt description of ftrace doc

Append and update the description about wakeup/wakeup_rt usage.

Link: http://lkml.kernel.org/r/1328695537-15081-2-git-send-email-geunsik.lim@gmail.com



Cc: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>