Added issue about flexbox lines and whether they should be real boxes or not.

Author: tabatkins

css3-flexbox/Overview.html

@@ -1286,6 +1286,9 @@ <h2 id=multi-line><span class=secno>6. </span> Multi-line Flexbox</h2>
    aligned within a flexbox with the &lsquo;<a href="#flex-line-pack0"><code
    class=property>flex-line-pack</code></a>&rsquo; property.
 
+  <p class=issue>Should flexbox lines be actual boxes in the box-tree, or
+   just a theoretical construct to help define things?
+
   <div class=example>
    <p>This example shows four buttons that do not fit horizontally.</p>
 

css3-flexbox/Overview.src.html

@@ -587,6 +587,8 @@ <h2 id='multi-line'>
 
 	<p>When additional lines are created, they are stacked in the flexbox in the <i>cross axis</i>.  Each line is completely independent; flexible lengths and the 'flex-pack' and 'flex-align' properties only pay attention to the items on a single line at a time.  The <i>main size</i> of a line is the same as the <i>main size</i> of the flexbox's content box.  The <i>cross size</i> of a line depends on whether the flexbox is <i>single-line</i> or <i>multi-line</i>: the <i>cross size</i> of the sole line in a <i>single-line</i> flexbox is the same as the <i>cross size</i> of the flexbox's content box, while the <i>cross size</i> of a line in a <i>multi-line</i> flexbox is the minimum size necessary to contain the <i>flexbox items</i> on the line, after aligning them with 'flex-align'.  The lines themselves are then aligned within a flexbox with the 'flex-line-pack' property.</p>
 
+	<p class='issue'>Should flexbox lines be actual boxes in the box-tree, or just a theoretical construct to help define things?</p>
+
 	<div class=example>
 		<p>This example shows four buttons that do not fit horizontally.</p>