Add notes to system_taskq

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #12771

module/os/freebsd/spl/spl_taskq.c

@@ -55,6 +55,11 @@ __FBSDID("$FreeBSD$");
 static uint_t taskq_tsd;
 static uma_zone_t taskq_zone;
 
+/*
+ * Global system-wide dynamic task queue available for all consumers. This
+ * taskq is not intended for long-running tasks; instead, a dedicated taskq
+ * should be created.
+ */
 taskq_t *system_taskq = NULL;
 taskq_t *system_delay_taskq = NULL;
 taskq_t *dynamic_taskq = NULL;