Add backcompat.md entry

Author: FBruzzesi

docs/backcompat.md

@@ -111,6 +111,10 @@ Which should you use? In general we recommend:
 
 ## `main` vs `stable.v1`
 
+- Since Narwhals 1.44:
+
+    - `LazyFrame.with_row_index` requires `order_by` to be specified as it is an order-dependent operation, in the main Narwhals namespace.
+
 - Since Narwhals 1.43:
 
     - `nw.get_level` is deprecated in the main Narwhals namespace.