add an explicit requirement of ordering of variables

specification/src/specification/queries/variables.md

@@ -20,4 +20,5 @@ The result contains one rowset containing articles from the author with ID `1`,
 ## Requirements
 
 - If `variables` are provided in the [`QueryRequest`](../../reference/types.md#queryrequest), then the [`QueryResponse`](../../reference/types.md#queryresponse) should contain one [`RowSet`](../../reference/types.md#rowset) for each set of variables.
+    - The ordering of [`RowSet`]s should EXACTLY match the ordering of the `variables` provided.
 - If `variables` are not provided, the data connector should return a single [`RowSet`](../../reference/types.md#rowset).