docs: Clarify xDS semantics for resources that do not exist. (#9322)
docs: Clarify xDS semantics for resources that do not exist. (#9322)

Clarify xDS protocol semantics for resources that do not exist.

Risk Level: Low
Testing: N/A
Docs Changes: Included in PR.

Signed-off-by: Mark D. Roth <f8bfa21f32be4966fb4ed6a329fe0dc16bf1b1c7@google.com>

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