Revert status message updates to revert back to exactly three status message states.

We still remove the part about spoofed status messages since it really is not part of the kernel message spec.

Author: jasongrout

docs/messaging.rst

@@ -1228,7 +1228,8 @@ Message type: ``status``::
     content = {
         # When the kernel starts to handle a message, it will enter the 'busy'
         # state and when it finishes, it will enter the 'idle' state.
-        execution_state : ('busy', 'idle', other optional states)
+        # The kernel will publish state 'starting' exactly once at process startup.
+        execution_state : ('busy', 'idle', 'starting')
     }
 
 When a kernel receives a request and begins processing it,
@@ -1239,10 +1240,6 @@ it shall publish a status message with ``execution_state: 'idle'``.
 Thus, the outputs associated with a given execution shall generally arrive
 between the busy and idle status messages associated with a given request.
 
-A kernel may send optional status messages with execution states other than
-`busy` or `idle`. For example, a kernel may send a status message with a
-`starting` execution state exactly once at process startup.
-
 .. note::
 
     **A caveat for asynchronous output**