Add javadoc to grpc codegen based on proto docs

Fixes #1612
diff --git a/compiler/src/testLite/proto/test.proto b/compiler/src/testLite/proto/test.proto
index fb2e3e7..d2a1fd5 100644
--- a/compiler/src/testLite/proto/test.proto
+++ b/compiler/src/testLite/proto/test.proto
@@ -24,6 +24,7 @@
 message StreamingOutputCallResponse {
 }
 
+// Test service that supports all call types.
 service TestService {
   // One request followed by one response.
   // The server returns the client payload as-is.