Refactor Mongoose documentation

PUBLISHED_FROM=e9a4e5c7b4a1d03b93a2a79e29de19e60e919929

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

@@ -0,0 +1,16 @@
+---
+title: "mg_get_http_var()"
+decl_name: "mg_get_http_var"
+symbol_kind: "func"
+signature: |
+  int mg_get_http_var(const struct mg_str *buf, const char *name, char *dst,
+                      size_t dst_len);
+---
+
+Fetches a HTTP form variable.
+
+Fetches a variable `name` from a `buf` into a buffer specified by `dst`,
+`dst_len`. The destination is always zero-terminated. Returns the length of
+a fetched variable. If not found, 0 is returned. `buf` must be valid
+url-encoded buffer. If destination is too small, `-1` is returned. 
+