doc: openthread: add section on message pool configuration

Add section on openthread message pool configuration.

Signed-off-by: Adrian Gielniewski <[email protected]>

doc/nrf/protocols/thread/configuring.rst

@@ -98,6 +98,22 @@ See the following files for more options that you might want to change:
 .. note::
    You can find the default configuration for all :ref:`openthread_samples` in the :file:`nrf/subsys/net/openthread/Kconfig.defconfig` file.
 
+.. _thread_configuring_messagepool:
+
+Message pool configuration
+**************************
+
+OpenThread uses a message pool to manage memory for message buffers.
+Memory for the message pool can be statically allocated by the OpenThread stack or managed by the platform.
+You can use the :kconfig:option:`CONFIG_OPENTHREAD_PLATFORM_MESSAGE_MANAGEMENT` Kconfig option to enable platform message management.
+
+Message buffer size and number of message buffers in the pool are be configured with the :kconfig:option:`CONFIG_OPENTHREAD_MESSAGE_BUFFER_SIZE` and :kconfig:option:`CONFIG_OPENTHREAD_NUM_MESSAGE_BUFFERS` Kconfig options, respectively.
+By default, both options are set to ``128``.
+
+.. note::
+   When using :ref:`thread_ug_prebuilt`, changing the :kconfig:option:`CONFIG_OPENTHREAD_PLATFORM_MESSAGE_MANAGEMENT` Kconfig option will have no effect.
+   Additionally, the :kconfig:option:`CONFIG_OPENTHREAD_MESSAGE_BUFFER_SIZE` Kconfig option has  to be set to the same value that is used in the pre-built library.
+
 .. _thread_ug_thread_specification_options:
 
 Thread Specification options