Add documentation about MDNS + FreeRTOS = crash (#1880)

See #1875

Author: earlephilhower

docs/wifi.rst

@@ -42,7 +42,9 @@ Please note that WiFi on the Pico W is a work-in-progress and there are some imp
 
 * Multicore is supported, but only core 0 may run ``WiFi`` related code.
 
-  * FreeRTOS is supported only on core 0 and from within ``setup`` and ``loop`, not tasks, due to the requirement for a very different LWIP implementation.  PRs always appreciated!
+  * FreeRTOS is supported only on core 0 and from within ``setup`` and ``loop``, not tasks, due to the requirement for a very different LWIP implementation.  PRs always appreciated!
+
+  * LEAmDNS (``MDNS``) is not supported in FreeRTOS due to internal IRQ-time memory allocations.
 
 The WiFi library borrows much work from the `ESP8266 Arduino Core <https://github.com/esp8266/Arduino>`__ , especially the ``WiFiClient`` and ``WiFiServer`` classes.