tracing/synthetic: Fix kernel-doc warnings in trace_events_synth.c

Fix kernel-doc warning:

kernel/trace/trace_events_synth.c:1257: warning: Function parameter
or member 'mod' not described in 'synth_event_gen_cmd_array_start'

Link: https://lkml.kernel.org/r/20230724140827.1023266-3-cuigaosheng1@huawei.com

Cc: <mhiramat@kernel.org>
Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>

kernel/trace/trace_events_synth.c

@@ -1230,6 +1230,7 @@ EXPORT_SYMBOL_GPL(__synth_event_gen_cmd_start);
  * synth_event_gen_cmd_array_start - Start synthetic event command from an array
  * @cmd: A pointer to the dynevent_cmd struct representing the new event
  * @name: The name of the synthetic event
+ * @mod: The module creating the event, NULL if not created from a module
  * @fields: An array of type/name field descriptions
  * @n_fields: The number of field descriptions contained in the fields array
  *