docs: fix incorrect doc about cluster warming in CDS (#4040)

Signed-off-by: Ruoshan Huang <ruoshan.huang@gmail.com>

Mirrored from https://github.com/envoyproxy/envoy @ 82e3541b0cd847724d5088e888ea90fc3f0cb418

XDS_PROTOCOL.md

@@ -236,10 +236,10 @@ xDS updates can be pushed independently if no new clusters/routes/listeners
 are added or if it's acceptable to temporarily drop traffic during
 updates. Note that in case of LDS updates, the listeners will be warmed
 before they receive traffic, i.e. the dependent routes are fetched through
-RDS if configured. On the other hand, clusters are not warmed when
+RDS if configured. Clusters are warmed when adding/removing/updating
-adding/removing/updating clusters. Similarly, routes are not warmed --
+clusters. On the other hand, routes are not warmed, i.e., the management
-i.e., the management plane must ensure that clusters referenced by a route
+plane must ensure that clusters referenced by a route are in place, before
-are in place, before pushing the updates for a rotue.
+pushing the updates for a route.
 
 ### Aggregated Discovery Services (ADS)