reflect/protoregistry: add link to namespace conflict FAQ

Updates golang/protobuf#1039.

Change-Id: Ia9f80da5dab1e6cd3cf9ad512b78b63b32065c31
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221782
Reviewed-by: Herbie Ong <herbie@google.com>
diff --git a/reflect/protoregistry/registry.go b/reflect/protoregistry/registry.go
index 4b5d8fb..4fc8158 100644
--- a/reflect/protoregistry/registry.go
+++ b/reflect/protoregistry/registry.go
@@ -31,8 +31,8 @@
 var ignoreConflict = func(d protoreflect.Descriptor, err error) bool {
 	log.Printf(""+
 		"WARNING: %v\n"+
-		"A future release will panic on registration conflicts.\n"+
-		// TODO(blocks): Add a URL pointing to documentation on how to resolve conflicts.
+		"A future release will panic on registration conflicts. See:\n"+
+		"https://developers.google.com/protocol-buffers/docs/reference/go/faq#namespace-conflict\n"+
 		"\n", err)
 	return true
 }