fix sentence implying that :first-letter and :before can be combined to say what the example's trying to demonstrate

Author: fantasai

selectors3/Overview.html

@@ -2291,9 +2291,9 @@ <h4><a name=gen-content>7.4. The ::before and ::after pseudo-elements</a></h4>
 href="#refsCSS21">[CSS21]</a>.</p>
 
 <p>When the <code>::first-letter</code> and <code>::first-line</code>
-pseudo-elements are combined with <code>::before</code> and
-<code>::after</code>, they apply to the first letter or line of the
-element including the inserted text.</p>
+pseudo-elements are applied to an element having content generated
+using <code>::before</code> or <code>::after</code>, they apply to the
+first letter or line of the element including the generated content.</p>
 
 <h2><a name=combinators>8. Combinators</a></h2>