Clarify nature of argument.

git-svn-id: svn+ssh://src.earth.threerings.net/nenya/trunk@315 ed5b42cb-e716-0410-a449-f6a68f950b19

src/as/com/threerings/flash/Animation.as

@@ -24,7 +24,8 @@ package com.threerings.flash {
 public interface Animation
 {
     /**
-     * The primary working method for your animation.
+     * The primary working method for your animation. The argument indicates how
+     * many milliseconds have passed since the previous update.
      */
     function updateAnimation (elapsed :Number) :void;
 }