doc: migration-guide: 3.6: IPv4 TTL and IPv6 hoplimit changes

Add information about IPv4 TTL and IPv6 hoplimit changes
between unicast and multicast packets.

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

doc/releases/migration-guide-3.6.rst

@@ -157,6 +157,10 @@ Networking
   a NULL pointer. IGMPv3 can be enabled using the Kconfig ``CONFIG_NET_IPV4_IGMPV3``.
   (:github:`65293`)
 
+* The network stack uses now separate IPv6 TTL (time-to-live) value for multicast packets. Before
+  the same TTL value was used for unicast and multicast packets. The IPv6 hop limit value is also
+  changed so that unicast and multicast packets can have different hop limit values.
+
 Other Subsystems
 ================