doc: migration guide v3.0: add steps to align with Fast Pair FMDN change

Added new content to the NCS v3.0 migration guide. The new
documentation describes how to align your Fast Pair application with
support for the Find My Device Network (FMDN) extension with the
recent changes to the FMDN API elements that are used to track the
FMDN provisioning state.

Ref: NCSDK-30856

Signed-off-by: Kamil Piszczek <[email protected]>

Author: kapi-no

doc/nrf/releases_and_maturity/migration/migration_guide_3.0.rst

@@ -91,6 +91,11 @@ Google Fast Pair
      The ``SB_CONFIG_BT_FAST_PAIR`` option no longer exists in this |NCS| release.
      Additionally, if you rely on the ``SB_CONFIG_BT_FAST_PAIR`` Kconfig option to set the :kconfig:option:`CONFIG_BT_FAST_PAIR` Kconfig option in the main image configuration of your application, you must align your main image configuration and set the :kconfig:option:`CONFIG_BT_FAST_PAIR` Kconfig option explicitly.
 
+   * If your Fast Pair application uses the Find My Device (FMD) extension, you must update your application code to correctly track the FMDN provisioning state.
+     From this |NCS| release, you must not rely on the :c:member:`bt_fast_pair_fmdn_info_cb.provisioning_state_changed` callback to report the initial provisioning state right after the Fast Pair module is enabled with the :c:func:`bt_fast_pair_enable` function call.
+     Instead, you must use the :c:func:`bt_fast_pair_fmdn_is_provisioned` function to initialize the FMDN provisioning state right after the :c:func:`bt_fast_pair_enable` function call.
+     For more details, see the :ref:`ug_bt_fast_pair_gatt_service_fmdn_info_callbacks_provisioning_state` section in the Fast Pair integration guide.
+
 nRF Cloud library
 -----------------