docs(guides): Add system tasks brief introduction

Author: donghengqaz

docs/en/api-guides/system-tasks.rst

@@ -0,0 +1,84 @@
+System Tasks
+************
+
+This document explains the ESP8266 RTOS SDK internal system tasks.
+
+Overview
+========
+
+The main tasks and their attributes are as following:
+
++-------------------------+----------------+----------------+
+| Names                   |   stack size   |    Priority    |
++=========================+================+================+
+| uiT                     |     3584(C)    |       14       |
++-------------------------+----------------+----------------+
+| IDLE                    |       768      |        0       |
++-------------------------+----------------+----------------+
+| Tmr                     |     2048(C)    |        2       |
++-------------------------+----------------+----------------+
+| ppT                     |     2048(C)    |       13       |
++-------------------------+----------------+----------------+
+| pmT                     |      1024      |       11       |
++-------------------------+----------------+----------------+
+| rtT                     |      2048      |       12       |
++-------------------------+----------------+----------------+
+| tiT                     |     2048(C)    |        8       |
++-------------------------+----------------+----------------+
+| esp_event_loop_task     |     2048(C)    |       10       |
++-------------------------+----------------+----------------+
+
+Note: (C) means it is configurable by "menuconfig".
+
+Tasks Introduction
+==================
+
+uiT
+---
+
+This task initializes the system, including peripherals, file system, user entry function and so on.
+This task will delete itself and free the resources after calling `app_main`.
+
+IDLE
+----
+
+This task is freeRTOS internal idle callback task, it is created when starting the freeRTOS.
+Its hook function is `vApplicationIdleHook`.
+The system's function of `sleep` and function of feeding `task watch dog` are called in the `vApplicationIdleHook`.
+
+Tmr
+---
+
+This task is the processor of freeRTOS internal software timer.
+
+ppT
+---
+
+This task is to process Wi-Fi hardware driver and stack. It posts messages from the logic link layer to the upper layer TCP/IP stack after transforming them into ethernet packets.
+
+pmT
+---
+
+The task is for system power management. It will check if the system can sleep right now, and if it is, it will start preparing for system sleep.
+
+rtT
+---
+
+The task is the processor of high priority hardware timer. It mainly process Wi-Fi real time events.
+It is suggested that functions based on this component should not be called in application, because it may block other low layer Wi-Fi functions.
+
+tiT
+---
+
+The task is the main task of TCP-IP stack(LwIP) , it is to deal with TCP-IP packets.
+
+esp_event_loop_task
+-------------------
+
+The task processes system events, for example, Wi-Fi and TCP-IP stack events.
+
+Suggestions
+===========
+
+In general, the priority of user task should NOT be higher than the system real timer task's priority (12). So it is suggested that keep your user tasks' priorities less than 12.  
+If you want to speed up the TCP/UDP throughput, you can try to set the priority of send/receive task to be higher than the "rtT" task's priority (8).