[web-animations-1] Clarify language describing optional arguments.

WebIDL uses not present/missing as opposed to not provided.

Author: birtles

web-animations-1/Overview.bs

@@ -3786,7 +3786,7 @@ interface Animation : EventTarget {
     1.  Let <var>animation</var> be a new {{Animation}} object.
     2.  Run the procedure to <a>set the timeline of an animation</a> on
         <var>animation</var> passing <var>timeline</var> as the <var>new
-        timeline</var> or, if a <var>timeline</var> argument is not provided,
+        timeline</var> or, if a <var>timeline</var> argument is missing,
         passing the <a>default document timeline</a> of the {{Document}}
         <a lt="document associated with a window">associated</a> with the
         {{Window}} that is the <a>current global object</a>.
@@ -3804,9 +3804,9 @@ interface Animation : EventTarget {
     :   <dfn argument
         for="Animation/Animation(effect, timeline)"
         lt="timeline">timeline</dfn>
-    ::  An optional value which, if provided, specifies the <a>timeline</a>
+    ::  An optional value which, if present, specifies the <a>timeline</a>
         with which to associate the newly-created <a>animation</a>.
-        If not provided, the <a>default document timeline</a> of the
+        If missing, the <a>default document timeline</a> of the
         {{Document}} <a lt="document associated with a window">associated</a>
         with the {{Window}} that is the <a>current global object</a> is used.