Fix mongoose docs generation

PUBLISHED_FROM=331821dcd1f7dc8a94581cd8a9b51aa00a89fddc

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

@@ -0,0 +1,13 @@
+---
+title: "mg_dns_insert_header()"
+decl_name: "mg_dns_insert_header"
+symbol_kind: "func"
+signature: |
+  int mg_dns_insert_header(struct mbuf *io, size_t pos,
+                           struct mg_dns_message *msg);
+---
+
+Inserts a DNS header to an IO buffer.
+
+Returns the number of bytes inserted. 
+