Clarify the status message.

1. Make the ‘starting’ status optional, since receiving it is unreliable anyway due to race conditions with connecting to the kernel.
2. Clarify that the kernel may send other status states that could be ignored
3. Take out the notebook-specific states, since that should be in notebook documentation, not in kernel messaging docs.

Author: jasongrout

docs/messaging.rst

@@ -1205,6 +1205,8 @@ Message type: ``error``::
 
     ``pyerr`` renamed to ``error``
 
+.. _status:
+
 Kernel status
 -------------
 
@@ -1215,8 +1217,7 @@ 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.
-        # The kernel will publish state 'starting' exactly once at process startup.
-        execution_state : ('busy', 'idle', 'starting')
+        execution_state : ('busy', 'idle', other optional states)
     }
 
 When a kernel receives a request and begins processing it,
@@ -1227,6 +1228,10 @@ 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**
@@ -1243,14 +1248,6 @@ between the busy and idle status messages associated with a given request.
     Busy and idle messages should be sent before/after handling every request,
     not just execution.
 
-.. note::
-
-    Extra status messages are added between the notebook webserver and websocket clients
-    that are not sent by the kernel. These are:
-
-    - restarting (kernel has died, but will be automatically restarted)
-    - dead (kernel has died, restarting has failed)
-
 Clear output
 ------------