Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

docs/c-api/http_server.h/mg_http_parse_header.md

@@ -0,0 +1,23 @@
+---
+title: "mg_http_parse_header()"
+decl_name: "mg_http_parse_header"
+symbol_kind: "func"
+signature: |
+  int mg_http_parse_header(struct mg_str *hdr, const char *var_name, char *buf,
+                           size_t buf_size);
+---
+
+Parses the HTTP header `hdr`. Finds variable `var_name` and stores its value
+in the buffer `buf`, `buf_size`. Returns 0 if variable not found, non-zero
+otherwise.
+
+This function is supposed to parse cookies, authentication headers, etc.
+Example (error handling omitted):
+
+    char user[20];
+    struct mg_str *hdr = mg_get_http_header(hm, "Authorization");
+    mg_http_parse_header(hdr, "username", user, sizeof(user));
+
+Returns the length of the variable's value. If buffer is not large enough,
+or variable not found, 0 is returned. 
+