[css-animations-2] Move paragraph and make generic

It describes a condition that isn't specific to that entry in the DL.

Author: fantasai

css-animations-2/Overview.bs

@@ -444,24 +444,23 @@ Declaring Animations</h2>
       specifies the length of time that an animation takes to complete one cycle.
       A negative <<time>> is invalid.
 
-      If the <<time>> is ''0s'', like the initial value,
-      the keyframes of the animation have no effect,
-      but the animation itself still occurs instantaneously.
-      Specifically, start and end events are fired;
-      if 'animation-fill-mode' is set to ''backwards'' or ''both'',
-      the first frame of the animation,
-      as defined by 'animation-direction',
-      will be displayed during the 'animation-delay'.
-      After the 'animation-delay' the last frame of the animation,
-      as defined by 'animation-direction',
-      will be displayed if 'animation-fill-mode' is set to ''forwards'' or ''both''.
-      If 'animation-fill-mode' is set to ''animation-fill-mode/none''
-      the animation will have no visible effect.
-
       For [=scroll-driven animations=],
       treated as ''animation-duration/auto''.
   </dl>
 
+  If the [=used value|used=] 'animation-duration' is ''0s'',
+  the keyframes of the animation have no effect,
+  but the animation itself still occurs instantaneously.
+  Specifically, start and end events are fired;
+  if 'animation-fill-mode' is set to ''backwards'' or ''both'',
+  the first frame of the animation,
+  as defined by 'animation-direction',
+  will be displayed during the 'animation-delay'.
+  After the 'animation-delay' the last frame of the animation,
+  as defined by 'animation-direction',
+  will be displayed if 'animation-fill-mode' is set to ''forwards'' or ''both''.
+  If 'animation-fill-mode' is set to ''animation-fill-mode/none''
+  the animation will have no visible effect.
 
 ## The 'animation-timing-function' property ## {#animation-timing-function}