message: Document incremental xDS semantics about resources that do not exist.
  Document incremental xDS semantics about resources that do not exist.
  Risk Level: Low
  Testing: N/A
  Docs Changes: Included in PR
  Release Notes: N/A
  This attempts to clarify the incremental xDS protocol semantics, as per discussion in
diff: @@ -687,6 +687,17 @@ names, which the server thought the client was already not subscribed
  to. The server must cleanly process such a request; it can simply ignore
  these phantom unsubscriptions.
  +Knowing When a Requested Resource Does Not Exist
  +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  +
  +When a resource subscribed to by a client does not exist, the server will send a :ref:`Resource
  +<envoy_api_msg_Resource>` whose :ref:`name <envoy_api_field_Resource.name>` field matches the
  +name that the client subscribed to and whose :ref:`resource <envoy_api_field_Resource.resource>`
  +field is unset. This allows the client to quickly determine when a resource does not exist without
  +waiting for a timeout, as would be done in the SotW protocol variants. However, clients are still
  +encouraged to use a timeout to protect against the case where the management server fails to send
  +a response in a timely manner.
  +
  REST-JSON polling subscriptions
  -------------------------------
  