doc: migration-guide-4.2: Add note about Bluetooth HCI changes

Mention the changes that have happened to the Bluetooth HCI driver
interface regarding packet type encoding.

Signed-off-by: Johan Hedberg <[email protected]>

Author: jhedberg

doc/releases/migration-guide-4.2.rst

@@ -272,6 +272,14 @@ Bluetooth Audio
 * ``CONFIG_BT_CSIP_SET_MEMBER_NOTIFIABLE`` has been renamed to
   :kconfig:option:`CONFIG_BT_CSIP_SET_MEMBER_SIRK_NOTIFIABLE``. (:github:`86763``)
 
+Bluetooth HCI
+=============
+
+* The buffer types passing through the HCI driver interface are now indicated as H:4 encoded prefix
+  bytes as part of the buffer payload itself. The bt_buf_set_type() and bt_buf_get_type() functions
+  have been deprecated, but are still usable, with the exception that they can only be
+  called once per buffer.
+
 Bluetooth Host
 ==============