Clarify text decorations in ::spelling-error and ::grammar-error

Issue w3c#9233
Add a note for authors on the requirements for custom text-decorations inside ::spelling-error pseudo (and grammar).

css-pseudo-4/Overview.bs

@@ -738,6 +738,15 @@ Default UA Styles</h3>
     :root::grammar-error  { text-decoration-line: grammar-error; }
   </pre>
 
+  Note: According to the 'text-decoration' specification, when the
+  'text-decoration-line' property is given with a value of 'spelling-error' or
+  'grammar-error', the UA is required to ignore other text-decoration
+  properties and render the default UA spelling or grammar highlight.
+  Given the recommended UA stylesheet properties above for '::spelling-error' and
+  '::grammar-error', this means you must explicitly provide a different
+  'text-decoration-line' value inside '::spelling-error' or '::grammar-error'
+   to render custom text decorations for spelling and grammar errors.
+
   Some [=highlight pseudo-elements=]
   should have <dfn>paired default highlight colors</dfn>--
   a default 'color' and 'background-color'
@@ -755,11 +764,6 @@ Default UA Styles</h3>
   or transitioning out a highlight style based on user interactions or timing.
   These are not controlled by CSS.
 
-  ISSUE: UA tweaks to the presentation of highlights
-  in ways that <em>are</em> controlled by CSS
-  are currently under discussion
-  in <a href='https://github.com/w3c/csswg-drafts/issues/6853'>Issue 6853</a>.
-
 <h4 id="paired-defaults">
 Paired Defaults</h4>