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

@@ -305,6 +305,13 @@ SPI
 Other subsystems
 ****************
 
+LoRa
+====
+
+* The :c:member:`lorawan_downlink_cb.cb` callback is now passed a pointer to the parent
+  :c:struct:`lorawan_downlink_cb` instance as the first parameter. This callback pointer can be used
+    to reference outer user-defined data structure using :c:macro:`CONTAINER_OF` macro.
+
 ZBus
 ====