clarify resource warming docs in xds protocol (#22377)

Signed-off-by: Rama Chavali <[email protected]>

docs/root/api-docs/xds_protocol.rst

@@ -678,6 +678,18 @@ server does not provide EDS/RDS responses, Envoy will not initialize
 itself during the initialization phase and the updates sent via CDS/LDS
 will not take effect until EDS/RDS responses are supplied.
 
+.. note::
+
+   Envoy specific implementation notes:
+
+   - Warming of ``Cluster`` is completed only when a new ``ClusterLoadAssignment``
+     response is supplied by management server even if there is no change in endpoints.
+   - Warming of ``Listener`` is completed even if management server does not send a
+     response for ``RouteConfiguration`` referenced by ``Listener``. Envoy will use the
+     previously sent ``RouteConfiguration`` to finish ``Listener`` warming. Management Server
+     has to send the ``RouteConfiguration`` response only if it has changed or it was never
+     sent in the past.
+
 .. _xds_protocol_eventual_consistency_considerations:
 
 Eventual consistency considerations