comment about delays

diff --git a/coders/png.c b/coders/png.c
index 613fc82..0ba7856 100644
--- a/coders/png.c
+++ b/coders/png.c
@@ -3951,6 +3951,9 @@
 #endif
     length;
 
+  /* These delays are all measured in image ticks_per_second,
+   * not in MNG ticks_per_second
+   */
   volatile size_t
     default_frame_delay,
     final_delay,