v1.15 backport Document RTL mode issue for fixed-wing approaches

Add warning about known issue with RTL mode for fixed-wing landings.

Author: hamishwillee

en/flight_modes_fw/return.md

@@ -25,6 +25,12 @@ The default type is recommended.
 
 :::
 
+::: warning
+There is a known issue ([PX4-Autopilot#25436](https://github.com/PX4/PX4-Autopilot/issues/25436)) with fixed-wing approaches and landings while in RTL mode.
+Please review the issue and verify in simulation that the behavior you get is safe in an RTL landing scenario (if not, consider using rally points).
+Note, any fix will not be backported to PX4 v1.15.
+:::
+
 ## Technical Summary
 
 Fixed-wing vehicles use the _mission landing/rally point_ return type by default.