reflect/protoreflect: mention bidirectional conversion of protodesc

Expand the mention of the protodesc package to indicate that it can
construct protoreflect.Descriptors from the descriptor messages, but
also convert a protoreflect.Descriptor back to the message form.

Change-Id: I9f3d75c6050fff603655ee0a4e8e29d0a2c0cf84
Reviewed-by: Damien Neil <>
diff --git a/reflect/protoreflect/proto.go b/reflect/protoreflect/proto.go
index 1b89bed..b669a4e 100644
--- a/reflect/protoreflect/proto.go
+++ b/reflect/protoreflect/proto.go
@@ -22,8 +22,9 @@
 // The protobuf descriptor interfaces are not meant to be implemented by
 // user code since they might need to be extended in the future to support
-// additions to the protobuf language. Protobuf descriptors can be constructed
-// using the "" package.
+// additions to the protobuf language.
+// The "" package converts between
+// google.protobuf.DescriptorProto messages and protobuf descriptors.
 // Go Type Descriptors