twn_util.h: clarify behavior of null result pointer

include/twn_util.h

@@ -110,6 +110,7 @@ _Alignas(16)
 
 
 /* calculates the overlap of two rectangles and places it in result. */
+/* result may be NULL. if this is the case, it will simply be ignored. */
 /* returns true if the rectangles are indeed intersecting. */
 TWN_API bool overlap_rect(const t_rect *a, const t_rect *b, t_rect *result);
 TWN_API bool overlap_frect(const t_frect *a, const t_frect *b, t_frect *result);