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>