[selectors] Apply the better :nth-child() text to :nth-last-child() as well.

Author: tabatkins

selectors/Overview.bs

@@ -2658,9 +2658,10 @@ Child-indexed Pseudo-classes</h3>
 '':nth-last-child()'' pseudo-class</h4>
 
 	The <dfn id='nth-last-child-pseudo' title=":nth-last-child()">:nth-last-child(<var>An+B</var> [of <var>sel</var>]? )</dfn>
-	pseudo-class notation represents an element that has <var>An+B</var>-1 siblings
-	<em>after</em> it in the document tree
-	which match the <a>selector list</a> <var>sel</var>.
+	pseudo-class notation represents the <var>An+B</var>th element
+	which matches the <a>selector list</a> <var>sel</var>
+	among its siblings,
+	counting backwards from the end.
 
 	The CSS Syntax Module [[!CSS3SYN]] defines the <a href="http://dev.w3.org/csswg/css-syntax/#anb"><var>An+B</var> notation</a>.
 	If <var>sel</var> is omitted,

selectors/Overview.html

@@ -2877,9 +2877,10 @@ <h4 class="heading settled" data-level=12.4.2 id=the-nth-last-child-pseudo><span
 <a class=css data-link-type=maybe href=#nth-last-child-pseudo title=:nth-last-child()>:nth-last-child()</a> pseudo-class</span><a class=self-link href=#the-nth-last-child-pseudo></a></h4>
 
 <p>The <dfn class=css data-dfn-type=selector data-export="" id=nth-last-child-pseudo title=:nth-last-child()>:nth-last-child(<var>An+B</var> [of <var>sel</var>]? )<a class=self-link href=#nth-last-child-pseudo></a></dfn>
-	pseudo-class notation represents an element that has <var>An+B</var>-1 siblings
-	<em>after</em> it in the document tree
-	which match the <a data-link-type=dfn href=#selector-list title="selector list">selector list</a> <var>sel</var>.</p>
+	pseudo-class notation represents the <var>An+B</var>th element
+	which matches the <a data-link-type=dfn href=#selector-list title="selector list">selector list</a> <var>sel</var>
+	among its siblings,
+	counting backwards from the end.</p>
 
 <p>The CSS Syntax Module <a data-biblio-type=normative data-link-type=biblio href=#biblio-css3syn title=CSS3SYN>[CSS3SYN]</a> defines the <a href=http://dev.w3.org/csswg/css-syntax/#anb><var>An+B</var> notation</a>.
 	If <var>sel</var> is omitted,