doc: net: Add link address change info to migration guide

Add information about "struct net_linkaddr" changes to 4.2
migration guide to help the developers to handle issues if
they access the net_linkaddr struct fields directly in their
code.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>

Author: jukkar

doc/releases/migration-guide-4.2.rst

@@ -145,6 +145,18 @@ Bluetooth Host
 Networking
 **********
 
+* The struct ``net_linkaddr_storage`` has been renamed to struct
+  :c:struct:`net_linkaddr` and the old struct ``net_linkaddr`` has been removed.
+  The struct :c:struct:`net_linkaddr` now contains space to store the link
+  address instead of having pointer that point to the link address. This avoids
+  possible dangling pointers when cloning struct :c:struct:`net_pkt`. This will
+  increase the size of struct :c:struct:`net_pkt` by 4 octets for IEEE 802.15.4,
+  but there is no size increase for other network technologies like Ethernet.
+  Note that any code that is using struct :c:struct:`net_linkaddr` directly, and
+  which has checks like ``if (lladdr->addr == NULL)``, will no longer work as expected
+  (because the addr is not a pointer) and must be changed to ``if (lladdr->len == 0)``
+  if the code wants to check that the link address is not set.
+
 Other subsystems
 ****************