web workflow: uses an mdns port service slot

Author: anecdata

docs/workflows.md

@@ -94,6 +94,10 @@ MDNS is used to resolve [`circuitpython.local`](http://circuitpython.local) to a
 hostname of the form `cpy-XXXXXX.local`. The `XXXXXX` is based on network MAC address. The device
 also provides the MDNS service with service type `_circuitpython` and protocol `_tcp`.
 
+Since port 80 (or the port assigned to `CIRCUITPY_WEB_API_PORT`) is used for MDNS, the `mdns`
+[module](https://docs.circuitpython.org/en/latest/shared-bindings/mdns/index.html#mdns.Server.advertise_service)
+can't also advertise a service on that port.
+
 ### HTTP
 The web server is HTTP 1.1 and may use chunked responses so that it doesn't need to precompute
 content length.