docs: add explanation for ACK and NACK semantics (#9309)
docs: add explanation for ACK and NACK semantics (#9309)

More precise explanation for ACK and NACK for implementers' benefit

Risk Level: Low (doc only change)
Testing: doc only testing (using github rst rendering)

Signed-off-by: Sanjay Pujare <a1166c2012c9126bcfeaa16a2368861d1d906236@users.noreply.github.com>

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