tracing: probes: Add a kerneldoc for traceprobe_parse_event_name()

Since traceprobe_parse_event_name() is a bit complicated, add a
kerneldoc for explaining the behavior.

Link: https://lore.kernel.org/all/175323430565.57270.2602609519355112748.stgit@devnote2/



Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>