[css-pseudo-4] Define what outline would mean if it was supported. Add an issue about whether it should be supported.

Author: fantasai

css-pseudo-4/Overview.bs

@@ -498,6 +498,13 @@ Styling Highlights</h3>
   applied to the text
   by the <a>originating element</a> and its ancestors and descendants.
 
+  The outline, if supported, must be drawn
+  around the union of the active portions of the <a>highlight overlay</a>
+  along the boundaries of those portions
+  and not between congruent parts.
+
+  ISSUE: Does anyone support 'outline' here? Is it wanted? Should we remove it from this list?
+
   Note: Historically (and at the time of writing)
   only 'color' and 'background-color' have been interoperably supported.