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/http.h/mg_send_head.md

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+---
+title: "mg_send_head()"
+decl_name: "mg_send_head"
+symbol_kind: "func"
+signature: |
+  void mg_send_head(struct mg_connection *n, int status_code,
+                    int64_t content_length, const char *extra_headers);
+---
+
+Send response line and headers.
+This function sends response line with the `status_code`, and automatically
+sends one header: either "Content-Length", or "Transfer-Encoding".
+If `content_length` is negative, then "Transfer-Encoding: chunked" header
+is sent, otherwise, "Content-Length" header is sent.
+
+NOTE: If `Transfer-Encoding` is `chunked`, then message body must be sent
+using `mg_send_http_chunk()` or `mg_printf_http_chunk()` functions.
+Otherwise, `mg_send()` or `mg_printf()` must be used.
+Extra headers could be set through `extra_headers` - and note `extra_headers`
+must NOT be terminated by a new line. 
+