docs: state the reduce/scan callback behavior change in the changelog

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Signed-off-by: feiwen zhu <mzhu@nvidia.com>

Author: azazhu

changelog.d/reduce-scan-isolation.md

@@ -1,3 +1,5 @@
 - Custom ``ct.reduce()`` and ``ct.scan()`` callbacks now keep captured scalar compile-time
-  constants inside their isolated bodies. Unsupported runtime and shaped captures are diagnosed
-  during cuTile Python compilation, before TileIR is invoked.
+  constants inside their own bodies, and every value inside a callback must be a scalar tile.
+  Capturing runtime values or non-scalar constants is rejected with a compile-time error.
+  Note: capturing runtime values in these callbacks previously compiled and is no longer
+  supported.