Add a new pseudo element search-text

Resolved from CSSWG

- w3c#10213 (comment)
- w3c#10212 (comment)

Author: jihyerish

css-pseudo-4/Overview.bs

@@ -1007,6 +1007,8 @@ Backgrounds</h4>
   over the ''::target-text'' overlay which is drawn
   over the ''::spelling-error'' overlay
   which is drawn over the ''::grammar-error'' overlay.
+  The ''::search-text'' overlay is drawn over or below the ''::selection'' overlay depending on UA, and
+  drawn over the the ''::target-text'' overlay.
 
   <wpt>
   css/css-pseudo/selection-overlay-and-grammar-001.html