WiFi off a boot: document a linker error when user wants to use wifi without including ESP8266WiFi.h (#7990)

Author: d-a-v

doc/esp8266wifi/generic-class.rst

@@ -61,6 +61,8 @@ linker).
 
 .. code:: cpp
 
+    #include <ESP8266WiFi.h>    // necessary to avoid a linker error
+
     void setup () {
     #ifdef WIFI_IS_OFF_AT_BOOT
         enableWiFiAtBootTime(); // can be called from anywhere with the same effect