doc: migration-guide-4.2: mention updated 'struct lorawan_downlink_cb'

This structure has updated callback, with additional parameter.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>

Author: mniestroj

doc/releases/migration-guide-4.2.rst

@@ -136,6 +136,14 @@ Networking
 Other subsystems
 ****************
 
+LoRa
+====
+
+* Struct :c:struct:`lorawan_downlink_cb` which is passeed to function
+  :c:func:`lorawan_register_downlink_callback` now accepts ``cb`` callback with additional
+  parameter, which is a pointer to callback structure itself. This callback pointer can be used to
+  reference outer user-defined data structure using ``CONTAINER_OF`` macro.
+
 Modules
 *******