docs: clarify xds docs with warming information (#6236)
docs: clarify xds docs with warming information (#6236)

As a follow-up to #6151 , this PR updates xds docs.

Signed-off-by: Rama Chavali <40109507d7c707c570c476fc1522e2d26df7f15e@salesforce.com>

jrajahalme/envoy,jrajahalme/envoy,lyft/envoy,lyft/envoy,envoyproxy/envoy,jrajahalme/envoy,eklitzke/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,istio/envoy,envoyproxy/envoy-wasm,istio/envoy,lyft/envoy,envoyproxy/envoy,istio/envoy,lizan/envoy,lyft/envoy,envoyproxy/envoy,istio/envoy,envoyproxy/envoy,envoyproxy/envoy,lyft/envoy,dnoe/envoy,envoyproxy/envoy,eklitzke/envoy,envoyproxy/envoy-wasm,istio/envoy,dnoe/envoy,eklitzke/envoy,istio/envoy,istio/envoy,envoyproxy/envoy,lizan/envoy,eklitzke/envoy,dnoe/envoy,envoyproxy/envoy-wasm,dnoe/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy-wasm,envoyproxy/envoy,lizan/envoy,dnoe/envoy,envoyproxy/envoy,jrajahalme/envoy,envoyproxy/envoy,lizan/envoy,lizan/envoy,jrajahalme/envoy,envoyproxy/envoy-wasm,envoyproxy/envoy,lyft/envoy,lizan/envoy,jrajahalme/envoy,envoyproxy/envoy,eklitzke/envoy