Add note about {{ layout }} to site.

site/_docs/variables.md

@@ -40,6 +40,16 @@ following is a reference of the available data.
 
       </p></td>
     </tr>
+    <tr>
+      <td><p><code>layout</code></p></td>
+      <td><p>
+
+        Layout specific information + the <a href="../frontmatter/">YAML front
+        matter</a>. Custom variables set via the YAML Front Matter in
+        layouts will be available here.
+
+      </p></td>
+    </tr>
     <tr>
       <td><p><code>content</code></p></td>
       <td><p>
@@ -326,6 +336,14 @@ following is a reference of the available data.
     <code>page.custom_css</code>.
 
   </p>
+  <p>
+
+    If you specify front matter in a layout, access that via <code>page</code>
+    For example, if you specify <code>class: full_page</code>
+    in a page’s front matter, that value will be available as
+    <code>layout.class</code> in the layout and its parents.
+
+  </p>
 </div>
 
 ## Paginator