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-on: https://go-review.googlesource.com/c/protobuf/+/221611
Reviewed-by: Damien Neil <dneil@google.com>
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 "google.golang.org/protobuf/reflect/protodesc" package.
+// additions to the protobuf language.
+// The "google.golang.org/protobuf/reflect/protodesc" package converts between
+// google.protobuf.DescriptorProto messages and protobuf descriptors.
 //
 //
 // Go Type Descriptors