Exclude gRPC generated code from javadoc processing

- part of ONOS-3323

Change-Id: Ie5975ace5e8ade2032eba726c59da2736502295b
diff --git a/docs/external-excludes b/docs/external-excludes
index 1890de1..8b8a3d5 100644
--- a/docs/external-excludes
+++ b/docs/external-excludes
@@ -52,3 +52,5 @@
 org.onosproject.mfwd*
 org.onosproject.mcast*
 org.onosproject.flowanalyzer
+org.onosproject.grpc*
+protobuf.*