message: docs: clarify xds docs with warming information
  As a follow-up to , this PR updates xds docs.
diff: @@ -151,7 +151,9 @@ For EDS/RDS, the management server does not need to supply every requested
  resource and may also supply additional, unrequested resources. `resource_names`
  is only a hint. Envoy will silently ignore any superfluous resources. When a
  requested resource is missing in a RDS or EDS update, Envoy will retain the last
  -known value for this resource. The management server may be able to infer all
  +known value for this resource except in the case where the `Cluster` or `Listener`
  +is being warmed. See [Resource warming](#resource-warming) section below on the expectations
  +during warming. The management server may be able to infer all
  the required EDS/RDS resources from the `node` identification in the
  `DiscoveryRequest`, in which case this hint may be discarded. An empty EDS/RDS
  `DiscoveryResponse` is effectively a nop from the perspective of the respective
  @@ -209,6 +211,20 @@ multiple `DiscoveryRequests` at a version until a new version is ready.
  An implication of the above resource update sequencing is that Envoy does not
  expect a `DiscoveryResponse` for every `DiscoveryRequest` it issues.
  +### Resource warming
  +
  +[`Clusters`](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/cluster_manager.html#cluster-warming)
  +and [`Listeners`](https://www.envoyproxy.io/docs/envoy/latest/configuration/listeners/lds#config-listeners-lds)
  +go through `warming` before they can serve requests. This process happens both during
  +[`Envoy initialization`](https://www.envoyproxy.io/docs/envoy/latest/intro/arch_overview/init.html#initialization)
  +and when the `Cluster` or `Listener` is updated. Warming of `Cluster` is completed only when a
  +`ClusterLoadAssignment` response is supplied by management server. Similarly, warming of `Listener`
  +is completed only when a `RouteConfiguration` is supplied by management server if the listener
  +refers to an RDS configuration. Management server is expected to provide the EDS/RDS updates during
  +warming. If management 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.
  +
  #### Eventual consistency considerations
  Since Envoy's xDS APIs are eventually consistent, traffic may drop briefly
  