[selectors-4][editorial] Fix the statement about wrapping a selector in :is(). w3c#6792

Author: tabatkins

selectors-4/Overview.bs

@@ -3704,7 +3704,9 @@ Grammar</h2>
 	<<forgiving-selector-list>> is used in '':is()'' and '':where()'' only.
 	Although it does have some minor implications on specificity,
 	wrapping a style rule's selector in '':is()''
-	effectively "upgrades" it to become forgiving.
+	effectively "upgrades" it to become forgiving,
+	so long as it doesn't contain any pseudo-elements
+	(which aren't valid in '':is()'' or '':where()'').
 
 	Syntactically, <<forgiving-selector-list>> is equivalent to <code><<any-value>>?</code>.
 	It is then [=parsed as a forgiving selector list=]