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>