Documentation: describe trace_buf_size parameter more accurately

I'm stuck into panic that too litte free memory is left when I boot with
trace_buf_size parameter. After digging into the problem, I found that
trace_buf_size is the size of trace buffer on each cpu rather than total
size of trace buffer. To prevent victim like me, change description of
trace_buf_size parameter more accurately.

Link: http://lkml.kernel.org/r/1417570760-10620-1-git-send-email-iamjoonsoo.kim@lge.com

Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

Documentation/kernel-parameters.txt

@@ -3477,7 +3477,7 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 			are saved.
 
 	trace_buf_size=nn[KMG]
-			[FTRACE] will set tracing buffer size.
+			[FTRACE] will set tracing buffer size on each cpu.
 
 	trace_event=[event-list]
 			[FTRACE] Set and start specified trace events in order