docs: resource hint explanation improvements in XDS_PROTOCOL.md. (#263)
docs: resource hint explanation improvements in XDS_PROTOCOL.md. (#263)

As suggested by @JonathanO.

Signed-off-by: Harvey Tuch <c185081708e02336640916420d0c772668570de5@google.com>
jrajahalme/envoy-api,lyft/envoy-api,lyft/envoy-api,lyft/envoy-api,rshriram/envoy-api,jrajahalme/envoy-api,jrajahalme/envoy-api,rshriram/envoy-api,jrajahalme/envoy-api,rshriram/envoy-api