[web-animations-1] Tweak descriptions of processing dictionary arguments

As of w3c#4235 these arguments will always be set.

The change to animate() simply corrects an existing oversight.

web-animations-1/Overview.bs

@@ -4725,16 +4725,11 @@ interface KeyframeEffect : AnimationEffect {
         :   If <var>options</var> is a {{KeyframeEffectOptions}} object,
         ::  Let <var>timing input</var> be <var>options</var>.
 
-        :   If <var>options</var> is a <code>double</code>,
+        :   Otherwise (if <var>options</var> is a <code>double</code>),
         ::  Let <var>timing input</var> be a new
             {{EffectTiming}} object with all members set to their default values
             and {{EffectTiming/duration}} set to <var>options</var>.
 
-        :   Otherwise (<var>options</var> is undefined),
-        ::  Let <var>timing input</var> be a new
-            {{EffectTiming}} object with all members set to their default
-            values.
-
     1.  Call the procedure to [=update the timing properties of an
         animation effect=] of |effect| from |timing input|.
 
@@ -5755,7 +5750,8 @@ dictionary GetAnimationsOptions {
         of the element on which this method was called as the
         <var>timeline</var> argument.
 
-    1.  Assign the value of the <code>id</code> member of <var>options</var> to
+    1.  If <var>options</var> is a {{KeyframeAnimationOptions}} object,
+        assign the value of the <code>id</code> member of <var>options</var> to
         <var>animation</var>'s {{Animation/id}} attribute.
 
     1.  Run the procedure to <a>play an animation</a> for