[css-display] display is not animatable (#2938)

Make it clear that display is not animatable (as opposed to supporting discrete animation.) Clarification discussed for other properties in #2751 (comment) Already tested in WPT web-animations/interfaces/KeyframeEffect/processing-a-keyframes-argument-001.html

Author: ewilligers

css-display-3/Overview.bs

@@ -165,7 +165,7 @@ Box Layout Modes: the 'display' property</h2>
 Applies to: all elements
 Inherited: no
 Computed value: see prose in a variety of specs
-Animatable: no
+Animation type:not animatable
 </pre>
 
 <p class=all-media>User Agents are expected to support this property on all media, including non-visual ones.</p>
@@ -1272,7 +1272,7 @@ SVG Elements {#unbox-svg}
 are ignored when rendering the contents.
 However, SVG [=presentation attributes=]--
 which map to CSS properties--
-continue to affect value processing and inheritance [[css-cascade-3]];
+continue to affect value processing and inheritance [[CSS3-CASCADE]];
 thus such attributes can affect
 the layout and visual formatting of the element’s descendants
 by influencing the values of such properties on those descendants.