docs(wifi): Add sniffer or promiscuous mode use limit

Author: donghengqaz

docs/en/api-reference/wifi/esp_wifi.rst

@@ -14,6 +14,12 @@ The WiFi libraries provide support for configuring and monitoring the ESP8266 Wi
 - Scanning for access points (active & passive scanning).
 - Promiscuous mode monitoring of IEEE802.11 WiFi packets.
 
+Important
+^^^^^^^^^
+
+Since the ESP8266 RTOS SDK V3.0, we moved some functions from IRAM to flash, including `malloc` and `free` fucntions, to save more memory.
+In this case, please do not read/write/erase flash during sniffer or promiscuous mode. You need to disable the sniffer or promiscuous mode at first, then read/write/erase flash.
+
 Application Examples
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