Merge branch 'doc/freertos' into 'master'

add a note about ESP-IDF FreeRTOS version

Closes DOC-315

See merge request espressif/esp-idf!9762

Author: krzychb

docs/en/api-reference/system/freertos.rst

@@ -6,6 +6,9 @@ Overview
 
 This section contains documentation of FreeRTOS types, functions, and macros. It is automatically generated from FreeRTOS header files.
 
+.. note::
+    ESP-IDF FreeRTOS is based on the Xtensa port of FreeRTOS v8.2.0, however some functions of FreeRTOS v9.0.0 have been backported. See the :ref:`backported-features` for more information.
+
 For more information about FreeRTOS features specific to ESP-IDF, see :doc:`ESP-IDF FreeRTOS SMP Changes<../../api-guides/freertos-smp>`
 and :doc:`ESP-IDF FreeRTOS Additions<freertos_additions>`.