DOC: sched: document the missing TASK_F_UEVT* flags

These are user-defined one-shot events that are application-specific
and reset upon wakeup and were not documented. No backport is needed
since these were added to 3.1.

doc/internals/api/scheduler.txt

@@ -208,6 +208,11 @@ state field before the call to ->process()
 
   - TASK_WOKEN_OTHER   any other application-defined wake-up reason.
 
+  - TASK_F_UEVT1       one-shot user-defined event type 1. This is application
+                       specific, and reset to 0 when the handler is called.
+
+  - TASK_F_UEVT2       one-shot user-defined event type 2. This is application
+                       specific, and reset to 0 when the handler is called.
 
 In addition, a few persistent flags may be observed or manipulated by the
 application, both for tasks and tasklets: