[nrf fromtree] doc: release: 3.3: Add note on MCUmgr handler update

Adds a note on MCUmgr Bluetooth and UDP transports now being
automatically registered without needing an application to
manually register them.

Signed-off-by: Jamie McCrae <[email protected]>
(cherry picked from commit 843f141)

Author: nordicjm

doc/releases/release-notes-3.3.rst

@@ -62,6 +62,18 @@ Changes in this release
   those registrations should be removed to prevent registering the same
   handler multiple times.
 
+* MCUmgr Bluetooth and UDP transports no longer need to be registered by the
+  application code, these will now automatically be registered at bootup (this
+  feature can be disabled or tweaked by adjusting:
+  :kconfig:option:`CONFIG_MCUMGR_TRANSPORT_BT_AUTOMATIC_INIT`,
+  :kconfig:option:`CONFIG_MCUMGR_TRANSPORT_BT_AUTOMATIC_INIT_WAIT`, and
+  :kconfig:option:`CONFIG_MCUMGR_TRANSPORT_UDP_AUTOMATIC_INIT`. If applications
+  register transports then those registrations should be removed to prevent
+  registering the same transport multiple times. If the Bluetooth stack needs
+  to be setup/initialised by another module or the application itself, then
+  :kconfig:option:`CONFIG_MCUMGR_TRANSPORT_BT_AUTOMATIC_INIT` should be
+  disabled and the application should call :c:func:`smp_bt_start` at startup.
+
 Removed APIs in this release
 ============================