chore: Update TableReference protobuf documentation (#6350)

This clarifies that `ticket` does not need to be an export ticket, and
that `batch_offset` will still work if it refers to an intermediate
operation which has been exported.

proto/proto-backplane-grpc/src/main/proto/deephaven/proto/table.proto

@@ -276,9 +276,13 @@ service TableService {
 
 message TableReference {
   oneof ref {
-    // an exported ticket
+    // A ticket to resolve to get the table. It's preferable to use export tickets in order to avoid races that are
+    // possible with tickets controlled by the server, but any ticket type will suffice as long as it resolves to a
+    // table.
     Ticket ticket = 1;
-    // an offset into a BatchRequest -- used to reference a result that will not be exported
+
+    // An offset into a BatchRequest's ops field, used to reference an intermediate operation which may not have been
+    // exported. Only valid to set when used in the context of a BatchRequest.
     sint32 batch_offset = 2;
   }
 }