rfc21: add missing depend event

Problem: A depend event that causes the job to transition
out of DEPEND state is discussed, and is present in the state
diagram, but is not defined.

Define the depend event, with empty context matching the current
implementation.

spec_21.rst

@@ -195,6 +195,20 @@ Example:
    {"timestamp":1552593348.073045,"name":"submit","context":{"priority":16,"userid":5588,"flags":0}}
 
 
+Depend Event
+^^^^^^^^^^^^
+
+All job dependencies have been met.
+
+No context is defined for this event.
+
+Example:
+
+.. code:: json
+
+    {"timestamp":1605115080.0358412,"name":"depend"}
+
+
 Priority Event
 ^^^^^^^^^^^^^^^^