First step towards separate userdata argument

PUBLISHED_FROM=93fe1cc2c5e2513509511a81e5919cbd37056b10

docs/c-api/net.h/mg_event_handler_t.md

@@ -4,7 +4,7 @@ decl_name: "mg_event_handler_t"
 symbol_kind: "typedef"
 signature: |
   typedef void (*mg_event_handler_t)(struct mg_connection *nc, int ev,
-                                     void *ev_data);
+                                     void *ev_data MG_UD_ARG(void *user_data));
 ---
 
 Callback function (event handler) prototype. Must be defined by the user.