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: NGeunsik Lim <geunsik.lim@samsung.com>
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

Documentation/trace/ftrace.txt

@@ -226,6 +226,13 @@ Here is the list of current tracers that may be configured.
 	Traces and records the max latency that it takes for
 	the highest priority task to get scheduled after
 	it has been woken up.
+        Traces all tasks as an average developer would expect.
+
+  "wakeup_rt"
+
+        Traces and records the max latency that it takes for just
+        RT tasks (as the current "wakeup" does). This is useful
+        for those interested in wake up timings of RT tasks.
 
   "hw-branch-tracer"