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>

Documentation/trace/events.txt

@@ -90,7 +90,8 @@ In order to facilitate early boot debugging, use boot option:
 
 	trace_event=[event-list]
 
-The format of this boot option is the same as described in section 2.1.
+event-list is a comma separated list of events. See section 2.1 for event
+format.
 
 3. Defining an event-enabled tracepoint
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