doc: migration-guide-4.1: Add note about default netmask value

Add an entry about a default IPv4 netmask value.

Signed-off-by: Robert Lubos <[email protected]>

Author: rlubos

doc/releases/migration-guide-4.1.rst

@@ -226,6 +226,10 @@ Networking
   :c:macro:`PROMETHEUS_HISTOGRAM_DEFINE` and :c:macro:`PROMETHEUS_SUMMARY_DEFINE`
   prototypes have changed. (:github:`81712`)
 
+* The default subnet mask on newly added IPv4 addresses is now specified with
+  :kconfig:option:`CONFIG_NET_IPV4_DEFAULT_NETMASK` option instead of being left
+  empty. Applications can still specify a custom netmask for an address with
+  :c:func:`net_if_ipv4_set_netmask_by_addr` function if needed.
 
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