reference a value from other linked table2

common/proto/tables_data.proto

@@ -258,6 +258,11 @@ message PutTableDataRequest {
   //   FAILED_PRECONDITION, instructing the caller to include X explicitly.
   //
   // Passing an empty map results in a no-op success response.
+  // A LINK may be supplied as {"id": "42", "version": "2"} to explicitly
+  // accept its target's current version, even when the row id is unchanged.
+  // Both values must be positive integers. The server rejects a stale version
+  // and atomically updates the saved FK version and all its stored linked values.
+  // A plain unchanged row id preserves the previously saved version and values.
   map<string, google.protobuf.Value> data = 4;
 
   // Required. Revision returned when this row was loaded or last saved.