Remove hide-implementation and update xds protocol doc (#502)

Signed-off-by: Rama <rama.rao@salesforce.com>

XDS_PROTOCOL.md

@@ -98,8 +98,10 @@ messages:
 The version provides Envoy and the management server a shared notion of the
 currently applied configuration, as well as a mechanism to ACK/NACK
 configuration updates. If Envoy had instead rejected configuration update __X__,
-it would reply with its previous version, which in this case was the empty
+it would reply with [`error_detail`] (https://www.envoyproxy.io/docs/envoy/latest/api-v2/api/v2/discovery.proto#envoy-api-field-discoveryrequest-error-detail)
-initial version:
+populated and its previous version, which in this case was the empty
+initial version. The error_detail has more details around the exact error message
+populated in the message field:
 
 ![No version update after NACK](diagrams/simple-nack.svg)
 

envoy/api/v2/discovery.proto

@@ -48,7 +48,6 @@ message DiscoveryRequest {
   // xDS implementations.
   string response_nonce = 5;
 
-  // [#not-implemented-hide:]
   // This is populated when the previous :ref:`DiscoveryResponse <envoy_api_msg_DiscoveryResponse>`
   // failed to update configuration. The *message* field in *error_details* provides the Envoy
   // internal exception related to the failure. It is only intended for consumption during manual