tracing: Make the documentation clear on trace_event boot option

Make it clear that event-list is a comma separated list of events.

Reported-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
LKML-Reference: <4B2F154C.2060503@cn.fujitsu.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>