doc: migration-guide: Mention socklen_t changes

Add information that the socklen_t was changed from size_t, which
can be either 32 or 64 bit long depending on system configuration,
to always be 32 bit long. This is now aligned to way Linux is defining
socklen_t.

Signed-off-by: Jukka Rissanen <[email protected]>

Author: jukkar

doc/releases/migration-guide-4.3.rst

@@ -189,6 +189,11 @@ Networking
   :c:macro:`HTTPS_SERVICE_DEFINE_EMPTY`, :c:macro:`HTTP_SERVICE_DEFINE` and
   :c:macro:`HTTPS_SERVICE_DEFINE`.
 
+* The size of socket address length type :c:type:`socklen_t` has changed. It is now defined to
+  be always 32 bit ``uint32_t`` in order to be aligned with Linux. Previously it was defined as
+  ``size_t`` which meant that the size could be either 32 bit or 64 bit depending on system
+  configuration.
+
 .. zephyr-keep-sorted-start re(^\w)
 
 .. zephyr-keep-sorted-stop