workqueue: doc: Add a note saturating the system_wq is not permitted

If something is expected to generate large number of concurrent works,
it should utilize its own dedicated workqueue rather than system wq.
Because this may saturate system_wq and potentially block other's works.
eg, cgroup release work. Let's document this as a note.

Signed-off-by: Chen Ridong <[e-mail]>
Signed-off-by: Tejun Heo <[e-mail]>

Documentation/core-api/workqueue.rst

···
   difference in execution characteristics between using a dedicated wq
   and a system wq.
 
+  Note: If something may generate more than @max_active outstanding
+  work items (do stress test your producers), it may saturate a system
+  wq and potentially lead to deadlock. It should utilize its own
+  dedicated workqueue rather than the system wq.
+
 * Unless work items are expected to consume a huge amount of CPU
   cycles, using a bound wq is usually beneficial due to the increased
   level of locality in wq operations and work item execution.