[css-pseudo-4] Switch UA stylesheet for ::spelling-error/::grammar-error/::target-text to recommendations rather than requirements.

Author: fantasai

css-pseudo-4/Overview.bs

@@ -560,7 +560,7 @@ Styling Highlights</h3>
 <h3 id=highlight-ua-styles>
 Default UA Styles</h3>
 
-  The following addition is made to the default UA stylesheet:
+  The following additions are recommended for the default UA stylesheet:
   <pre><code class="lang-css">
     /* Represent default spelling/grammar error styling in an adjustable way */
     :root::spelling-error { text-decoration-line: spelling-error; }