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/struct_mg_dns_message.md

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+---
+title: "struct mg_dns_message"
+decl_name: "struct mg_dns_message"
+symbol_kind: "struct"
+signature: |
+  struct mg_dns_message {
+    struct mg_str pkt; /* packet body */
+    uint16_t flags;
+    uint16_t transaction_id;
+    int num_questions;
+    int num_answers;
+    struct mg_dns_resource_record questions[MG_MAX_DNS_QUESTIONS];
+    struct mg_dns_resource_record answers[MG_MAX_DNS_ANSWERS];
+  };
+---
+
+DNS message (request and response). 
+