Fix mongoose docs generation

PUBLISHED_FROM=331821dcd1f7dc8a94581cd8a9b51aa00a89fddc

docs/c-api/mg_dns.h/mg_set_protocol_dns.md

@@ -0,0 +1,19 @@
+---
+title: "mg_set_protocol_dns()"
+decl_name: "mg_set_protocol_dns"
+symbol_kind: "func"
+signature: |
+  void mg_set_protocol_dns(struct mg_connection *nc);
+---
+
+Attaches a built-in DNS event handler to the given listening connection.
+
+The DNS event handler parses the incoming UDP packets, treating them as DNS
+requests. If an incoming packet gets successfully parsed by the DNS event
+handler, a user event handler will receive an `MG_DNS_REQUEST` event, with
+`ev_data` pointing to the parsed `struct mg_dns_message`.
+
+See
+[captive_dns_server](https://github.com/cesanta/mongoose/tree/master/examples/captive_dns_server)
+example on how to handle DNS request and send DNS reply. 
+