Update src/unix/lwt_unix.h

Author: raphael-proust

src/unix/lwt_unix.h

@@ -14,6 +14,9 @@
 #include <caml/socketaddr.h>
 #include <string.h>
 
+// The following macro is for backwards compatibility.
+// It is given an `lwt_` prefix to avoid name collisions for code which
+// include both this file and alloc.h.
 #if OCAML_VERSION < 50000
 #define lwt_convert_flag_list(flags, table) \
     caml_convert_flag_list((flags), (int *)(table))