Generate document for proto service

[lambdai]

Description:

proto file contains both message and service. Currently only the comment associated with message or field in the message is generated.

• example of service and message

$ egrep -B 2 "^message|^service" api/envoy/api/v2/cds.proto |head -10

// Return list of all clusters this proxy will load balance to.
service ClusterDiscoveryService {
--
// Configuration for a single upstream cluster.
// [#comment:next free field: 39]
message Cluster {
--
// An extensible structure containing the address Envoy should bind to when
// establishing upstream connections.

• message doc

$ grep "An extensible" generated/ -rn
generated/rst/api-v2/api/v2/cds.proto.rst:1072:An extensible structure containing the address Envoy should bind to when
generated/docs/api-v2/api/v2/cds.proto.html:985:<p>An extensible structure containing the address Envoy should bind to when
generated/docs/_sources/api-v2/api/v2/cds.proto.rst.txt:1072:An extensible structure containing the address Envoy should bind to when
Binary file generated/docs/.doctrees/api-v2/api/v2/cds.proto.doctree matches

• service doc: none

$ grep "Return list of all clusters" generated/ -rn

[mattklein123]

Thanks for tracking this down @silentdai. This is a duplicate of #3091 so let's track there.