docs: add code snippet for checking OTA state

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

@@ -36,6 +36,30 @@ The main purpose of the application rollback is to keep the device working after
 * The application has critical errors and further work is not possible, a rollback to the previous application is required, :cpp:func:`esp_ota_mark_app_invalid_rollback_and_reboot` marks the running application with the state ``ESP_OTA_IMG_INVALID`` and reset. This application will not be selected by the bootloader for boot and will boot the previously working application.
 * If the :ref:`CONFIG_BOOTLOADER_APP_ROLLBACK_ENABLE` option is set, and a reset occurs without calling either function then the application is rolled back.
 
+The following code serves detect the initial boot for an application after the OTA update. Upon the first boot, the application checks its state and performs diagnostics. If the diagnostics are successful, the application should call :cpp:func:`esp_ota_mark_app_valid_cancel_rollback` to confirm the operability of the application. If the diagnostics fail, the application should call :cpp:func:`esp_ota_mark_app_invalid_rollback_and_reboot` to roll back to the previous working application.
+
+If the application is not able to boot or execute this code due to an abort/reboot/power loss error, the bootloader marks this application as ``ESP_OTA_IMG_INVALID`` in the next booting attempt and rolls back to the previous working application.
+
+.. code:: c
+
+    const esp_partition_t *running = esp_ota_get_running_partition();
+    esp_ota_img_states_t ota_state;
+    if (esp_ota_get_state_partition(running, &ota_state) == ESP_OK) {
+        if (ota_state == ESP_OTA_IMG_PENDING_VERIFY) {
+            // run diagnostic function ...
+            bool diagnostic_is_ok = diagnostic();
+            if (diagnostic_is_ok) {
+                ESP_LOGI(TAG, "Diagnostics completed successfully! Continuing execution ...");
+                esp_ota_mark_app_valid_cancel_rollback();
+            } else {
+                ESP_LOGE(TAG, "Diagnostics failed! Start rollback to the previous version ...");
+                esp_ota_mark_app_invalid_rollback_and_reboot();
+            }
+        }
+    }
+
+For the example incorporating the above code snippet, see the :example:`system/ota/native_ota_example` example.
+
 .. note::
 
   The state is not written to the binary image of the application but rather to the ``otadata`` partition. The partition contains a ``ota_seq`` counter, which is a pointer to the slot (``ota_0``, ``ota_1``, ...) from which the application will be selected for boot.