doc: releases: Add release note entry for OpenThread update

Add an entry to the release note to inform about the new
CONFIG_OPENTHREAD_SYS_INIT Kconfig option.

Signed-off-by: Arkadiusz Balys <arkadiusz.balys@nordicsemi.no>

Author: ArekBalysNordic

doc/connectivity/networking/api/thread.rst

@@ -64,4 +64,69 @@ Zephyr's OpenThread L2 platform adaptation layer glues the external OpenThread
 stack together with Zephyr's IEEE 802.15.4 protocol agnostic driver API. This
 API is of interest to OpenThread L2 **subsystem contributors** only.
 
+OpenThread Platform API
+=======================
+
+The OpenThread platform API is defined by the OpenThread stack and implemented in Zephyr as an
+OpenThread module. Applications can use this implementation directly, or access it through the
+OpenThread L2 adaptation layer.
+
+Using the OpenThread L2 Adaptation Layer API
+--------------------------------------------
+
+To use the OpenThread platform API via the OpenThread L2 adaptation layer, enable both the
+:kconfig:option:`CONFIG_NET_L2_OPENTHREAD` and :kconfig:option:`CONFIG_NETWORKING` Kconfig options
+by setting them to ``y``. The adaptation layer will use the OpenThread radio API implementation
+found in :file:`modules/openthread/platform/radio.c`. In this setup, the OpenThread stack is
+initialized and managed by the adaptation layer.
+
+Using the OpenThread Platform API Directly
+------------------------------------------
+
+You can also use the OpenThread platform API directly, bypassing the OpenThread L2 adaptation
+layer. However, this approach requires you to provide your own implementation of the OpenThread
+radio API that is compatible with your specific radio driver.
+
+To use the OpenThread platform API directly, set the :kconfig:option:`CONFIG_OPENTHREAD` Kconfig
+option to ``y``, and do **not** set :kconfig:option:`CONFIG_NET_L2_OPENTHREAD`. In this case, you
+must implement the following functions from the `OpenThread radio API
+<https://openthread.io/reference/group/radio-config>`_ using your own radio driver:
+
+* ``otPlatRadioGetPromiscuous``
+* ``otPlatRadioGetCcaEnergyDetectThreshold``
+* ``otPlatRadioGetTransmitPower``
+* ``otPlatRadioGetIeeeEui64``
+* ``otPlatRadioSetPromiscuous``
+* ``otPlatRadioGetCaps``
+* ``otPlatRadioGetTransmitBuffer``
+* ``otPlatRadioSetPanId``
+* ``otPlatRadioEnable``
+* ``otPlatRadioDisable``
+* ``otPlatRadioReceive``
+* ``otPlatRadioGetRssi``
+* ``otPlatRadioGetReceiveSensitivity``
+* ``otPlatRadioEnergyScan``
+* ``otPlatRadioSetExtendedAddress``
+* ``otPlatRadioSetShortAddress``
+* ``otPlatRadioAddSrcMatchExtEntry``
+* ``otPlatRadioTransmit``
+* ``otPlatRadioClearSrcMatchShortEntries``
+* ``otPlatRadioClearSrcMatchExtEntries``
+* ``otPlatRadioEnableSrcMatch``
+* ``otPlatRadioAddSrcMatchShortEntry``
+* ``otPlatRadioClearSrcMatchShortEntry``
+* ``otPlatRadioClearSrcMatchExtEntry``
+
+Additionally, you must implement the following functions from the OpenThread radio API (see
+:zephyr_file:`include/zephyr/net/openthread.h`) to handle radio initialization and event processing:
+
+* :c:func:`platformRadioInit`
+* :c:func:`platformRadioProcess`
+
+To initialize the OpenThread stack in this approach, either call the :c:func:`ot_platform_init`
+function in your application, or enable the :kconfig:option:`CONFIG_OPENTHREAD_SYS_INIT` Kconfig
+option to automatically initialize OpenThread during system startup. You can set the
+initialization priority using the :kconfig:option:`CONFIG_OPENTHREAD_SYS_INIT_PRIORITY` Kconfig
+option.
+
 .. doxygengroup:: openthread

doc/releases/release-notes-4.2.rst

@@ -175,12 +175,12 @@ New APIs and options
 
   * OpenThread
 
-    * Moved OpenThread-related Kconfig options from ``subsys/net/l2/openthread/Kconfig`` to
-      ``modules/openthread/Kconfig``.
-    * OpenThread has been refactored into a standalone module, decoupled from Zephyr's networking
-      stack. Most old APIs have been deprecated and the new APIs are used by the Zephyr L2 layer to
-      maintain backward compatibility. For migration steps and details, see the
+    * Moved OpenThread-related Kconfig options from :zephyr_file:`subsys/net/l2/openthread/Kconfig`
+      to :zephyr_file:`modules/openthread/Kconfig`.
+    * Refactored OpenThread networking API, see the OpenThread section of the
       :ref:`migration guide <migration_4.2>`.
+    * :kconfig:option:`CONFIG_OPENTHREAD_SYS_INIT`
+    * :kconfig:option:`CONFIG_OPENTHREAD_SYS_INIT_PRIORITY`
 
   * zperf