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>
diff --git a/Documentation/trace/events.txt b/Documentation/trace/events.txt
index 02ac6ed..778ddf3 100644
--- a/Documentation/trace/events.txt
+++ b/Documentation/trace/events.txt
@@ -90,7 +90,8 @@
 
 	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
 =======================================