demos: remove inconsistent readme

demos/benchmark/lv_demo_benchmark.h

@@ -29,9 +29,25 @@ extern "C" {
  * GLOBAL PROTOTYPES
  **********************/
 
-/** Run all test scenes in the LVGL benchmark with a given mode
+/**
+ * Run all benchmark scenes.
+ *
+ * On the summary end screen the values shall be interpreted according to the followings:
+ * - CPU usage:
+ *    - If `LV_SYSMON_GET_IDLE` is not modified it's measured based on the time spent in
+ *      `lv_timer_handler`.
+ *    - If an (RT)OS is used `LV_SYSMON_GET_IDLE` can be changed to a custom function
+ *      which returns the idle percentage of idle task.
+ *
+ * - FPS: LVGL attempted to render this many times in a second. It's limited based on `LV_DEF_REFR_PERIOD`
+ *
+ * - Render time: LVGL spent this much time with rendering only. It's not aware of task yielding,
+ *   but simply the time difference between the start and end of the rendering is measured
+ *
+ * - Flush time: It's the sum of
+ *     - the time spent in the `fluch_cb` and
+ *     - the time spent with waiting for flush ready.
  */
-
 void lv_demo_benchmark(void);
 
 /**********************