[selectors] Rephrase :nth-child() definition to work better with the sel argument.

--HG--
extra : rebase_source : 71115b137d5a68999142ae0abda31a2160867b36

Author: tabatkins

selectors/Overview.bs

@@ -105,25 +105,25 @@ Selectors Overview</h2>
 			<tr>
 				<td><code>*</code>
 				<td>any element
-				<td><a href="#universal-selector">Universal selector</a>
+				<td><a href="#the-universal-selector" section>Universal selector</a>
 				<td>2
 			<tr>
 				<td><code>E</code>
 				<td>an element of type E
-				<td><a href="#type-selectors">Type (tag name) selector</a>
+				<td><a href="#type-selectors" section>Type (tag name) selector</a>
 				<td>1
 		<tbody>
 			<tr>
 				<td><code>E:not(<var>s1</var>, <var>s2</var>)</code>
 				<td>an E element that does not match either <a>compound selector</a> <var>s1</var>
 						or <a>compound selector</a> <var>s2</var>
-				<td><a href="#negation">Negation pseudo-class</a>
+				<td><a href="#negation" section>Negation pseudo-class</a>
 				<td>3/4
 			<tr>
 				<td><code>E:matches(<var>s1</var>, <var>s2</var>)</code>
 				<td>an E element that matches <a>compound selector</a> <var>s1</var>
 						and/or <a>compound selector</a> <var>s2</var>
-				<td><a href="#matches">Matches-any pseudo-class</a>
+				<td><a href="#matches" section>Matches-any pseudo-class</a>
 				<td>4
 			<tr>
 				<td><code>E:has(<var>rs1</var>, <var>rs2</var>)</code>
@@ -2576,9 +2576,9 @@ Child-indexed Pseudo-classes</h3>
 '':nth-child()'' pseudo-class</h4>
 
 	The <dfn id='nth-child-pseudo' title=":nth-child()">:nth-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>before</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.
 
 	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,
@@ -2604,14 +2604,14 @@ Child-indexed Pseudo-classes</h3>
 		For example, the following selector matches the first three “important” list items,
 		denoted by the ''.important'' class:
 
-		<pre>li:nth-child(-n+3 of .important)</pre>
+		<pre>:nth-child(-n+3 of li.important)</pre>
 
 		Note that this is different from moving the selector outside of the function, like:
 
 		<pre>li.important:nth-child(-n+3)</pre>
 
-		This selector instead just selects the first three list items
-		if they also have the ''.important'' class.
+		This selector instead just selects the first three children
+		if they also happen to be "important" list items.
 	</div>
 
 	<div class="example">