[web-animations-1] Clarify handling of "auto" value for duration

Closes #2471.

Author: birtles

web-animations-1/Overview.bs

@@ -4306,9 +4306,15 @@ dictionary OptionalEffectTiming {
     effect</a>.
 
     In this level of this specification, the string value <code>auto</code>
-    is equivalent to zero.
+    is treated as the value zero for the purpose of timing model calculations
+    and for the result of the {{EffectTiming/duration}} member returned
+    from {{AnimationEffect/getComputedTiming()}}.
+    If the author specifies the <code>auto</code> value user agents must,
+    however, return <code>auto</code> for the {{EffectTiming/duration}} member
+    returned from {{AnimationEffect/getTiming()}}.
+
     This is a forwards-compatiblity measure since a future level of this
-    specification will introduce group effects where the
+    specification is expected to introduce group effects where the
     <code>auto</code> value expands to include the duration of the child
     effects.