nlj upgrade guide (apache#17202)

Author: 2010YOUY01

docs/source/library-user-guide/upgrading.md

@@ -188,6 +188,16 @@ See [#17028] for more details and an example implementation for `SortExec`.
 
 [#17028]: https://github.com/apache/datafusion/pull/17028
 
+### Nested Loop Join input sort order cannot be preserved
+
+The Nested Loop Join operator has been rewritten from scratch to improve performance and memory efficiency. From the micro-benchmarks: this change introduces up to 5X speed-up and uses only 1% memory in extreme cases compared to the previous implementation.
+
+However, the new implementation cannot preserve input sort order like the old version could. This is a fundamental design trade-off that prioritizes performance and memory efficiency over sort order preservation.
+
+See [#16996] for details.
+
+[#16996]: https://github.com/apache/datafusion/pull/16996
+
 ## DataFusion `49.0.0`
 
 ### `MSRV` updated to 1.85.1