doc: migration-guide: 3.5: Add note on SMP version 2 error handlers

Adds a note about required changed for Zephyr 3.4 SMP version 2
error translation handlers in Zephyr 3.5 (or newer)

Signed-off-by: Jamie McCrae <[email protected]>

Author: nordicjm

doc/releases/migration-guide-3.5.rst

@@ -34,6 +34,13 @@ Required changes
   SMP version 2 error code defines for in-tree modules have been updated to
   replace the ``*_RET_RC_*`` parts with ``*_ERR_*``.
 
+* MCUmgr SMP version 2 error translation (to legacy MCUmgr error code) is now
+  handled in function handlers by setting the ``mg_translate_error`` function
+  pointer of :c:struct:`mgmt_group` when registering a group. See
+  :c:type:`smp_translate_error_fn` for function details. Any SMP version 2
+  handlers made for Zephyr 3.4 need to be updated to include these translation
+  functions when the groups are registered.
+
 * ``zephyr,memory-region-mpu`` was renamed ``zephyr,memory-attr`` and its type
   moved from 'enum' to 'int'. To have a seamless conversion this is the
   required change in the DT: