Reworded opening of the flex() section to actually describe what flexibility does.

Author: tabatkins

css3-flexbox/Overview.html

@@ -1053,10 +1053,11 @@ <h2 id=flexibility><span class=secno>4. </span> Flexibility: the
 
   <p>The defining aspect of flexbox layout is the ability to make the <a
    href="#flexbox-item"><i>flexbox items</i></a> "flex", altering their width
-   or height to fill the available space. This is done by setting an
+   or height to fill the available space. This is done by using a &lsquo;<a
+   href="#ltflex"><code class=css>&lt;flex></code></a>&rsquo; value in the
    element's &lsquo;<code class=property>width</code>&rsquo; or &lsquo;<code
-   class=property>height</code>&rsquo; to a <a
-   href="#flexible-length"><i>flexible length</i></a> value.
+   class=property>height</code>&rsquo; property. A flexbox distributes free
+   space proportionally to its items based on their flexibility.
 
   <p>The &lsquo;<code class=css>flex()</code>&rsquo; notation is used to
    specify the parameters of a <dfn id=flexible-length

css3-flexbox/Overview.src.html

@@ -498,7 +498,7 @@ <h3 id='flex-order'>
 <h2 id='flexibility'>
 Flexibility: the ''flex()'' notation</h2>
 
-	<p>The defining aspect of flexbox layout is the ability to make the <i>flexbox items</i> "flex", altering their width or height to fill the available space.  This is done by setting an element's 'width' or 'height' to a <i>flexible length</i> value.</p>
+	<p>The defining aspect of flexbox layout is the ability to make the <i>flexbox items</i> "flex", altering their width or height to fill the available space.  This is done by using a ''&lt;flex>'' value in the element's 'width' or 'height' property.  A flexbox distributes free space proportionally to its items based on their flexibility.</p>
 
 	<p>The ''flex()'' notation is used to specify the parameters of a <dfn title="flexible length|flexible lengths|flexible length's">flexible length</dfn>: the <dfn id="positive-flexibility" title="positive flexibility">positive</dfn> and <dfn>negative flexibility</dfn>, and the <dfn>preferred size</dfn>.  The syntax of the ''flex()'' notation is:</p>
 
@@ -601,8 +601,6 @@ <h3 id='flex-pack'>
 		<p class='caption'>An illustration of the five 'flex-pack' keywords and their effects on a flexbox with three colored items.</p>
 	</div>
 
-	<p class='note'>A previous revision of this spec allowed margins to flex directly, which allowed an effect similar to 'flex-pack'.  In particular, it allowed an author to, for example, split a flexbox in half, with some of the items pushed toward the start and the rest pushed toward the end, by flexing exactly one margin.  This sort of effect is no longer possible without either using the ::before or ::after pseudoelements or adding additional elements to the document, to act as an empty item that can then flex.  It's expected that we will develop something in the future to make this easier, such as a more general pseudoelement or perhaps the ability to explicitly control individual spaces between flexbox items.</p>
-
 
 <h3 id='flex-align'>
 Cross Axis Alignment: the 'flex-align' and 'flex-item-align' properties</h3>