[css-align] Add section to explain that justify-self doesn't apply to flex items.

Author: tabatkins

css-align/Overview.html

@@ -1074,6 +1074,17 @@ <h3 id=justify-self-property><span class=secno>5.1. </span>
      absolutely-positioned boxes. (This is because CSS 2.1 does not stretch
      replaced elements to fit into fixed offsets.)
 
+   <dt>Flex Items:
+
+   <dd>
+    <p>This property does not apply to <i>flex items</i>, because there is
+     more than one item in the <i>main axis</i>. See ‘<code
+     class=property>flex</code>’ for stretching and ‘<a
+     href="#justify-content"><code
+     class=property>justify-content</code></a>’ for <i>main-axis</i>
+     alignment. <a href="#CSS3-FLEXBOX"
+     rel=biblioentry>[CSS3-FLEXBOX]<!--{{!CSS3-FLEXBOX}}--></a>
+
    <dt>Grid Items:
 
    <dd>

css-align/Overview.src.html

@@ -597,6 +597,12 @@ <h3 id='justify-self-property'>
         The ''stretch'' keyword is equivalent to ''start'' on replaced absolutely-positioned boxes.
         (This is because CSS 2.1 does not stretch replaced elements to fit into fixed offsets.)
 
+    <dt>Flex Items:
+    <dd>
+      <p>This property does not apply to <i>flex items</i>,
+        because there is more than one item in the <i>main axis</i>.
+        See 'flex' for stretching and 'justify-content' for <i>main-axis</i> alignment. [[!CSS3-FLEXBOX]]
+
     <dt>Grid Items:
     <dd>
       <p>The 'justify-self' property applies along the grid's <i>row axis</i>.