doc: clarify that trace_events= takes a comma-separated list

It took me browsing through the source code to determine that I was,
indeed, using the wrong delimiter in my command lines. So I might as
well document it for the next person.
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/kernel-parameters.txt

@@ -3992,8 +3992,9 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
 
 	trace_event=[event-list]
 			[FTRACE] Set and start specified trace events in order
-			to facilitate early boot debugging.
-			See also Documentation/trace/events.txt
+			to facilitate early boot debugging. The event-list is a
+			comma separated list of trace events to enable. See
+			also Documentation/trace/events.txt
 
 	trace_options=[option-list]
 			[FTRACE] Enable or disable tracer options at boot.