Docs are generated for Mongoose

Comments in headers are changed a bit: removed adoc-specific stuff,
markdown is used instead

PUBLISHED_FROM=9242cce85cc52a47a197d377e7e23804721a6bb5

docs/c-api/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);
+---
+
+Insert a DNS header to an IO buffer.
+
+Return number of bytes inserted. 
+