title: What does the "update" element do in Magento layout XML?
body: <p>Right now I'm exploring the internals of the admin section of Magento and I stumbled on this piece of XML:</p>
  
  <p>File: <code>app/design/adminhtml/default/default/layout/catalog.xml</code>, around line 55</p>
  
  <pre><code>50            &lt;block type="core/template" template="catalog/wysiwyg/js.phtml"/&gt;
  51        &lt;/reference&gt;
  52    &lt;/adminhtml_catalog_product_new&gt;
  53    
  54    &lt;adminhtml_catalog_product_edit&gt;
  55        &lt;update handle="editor"/&gt;
  56        &lt;reference name="content"&gt;
  57            &lt;block type="adminhtml/catalog_product_edit" name="product_edit"&gt;&lt;/block&gt;
  58        &lt;/reference&gt;
  </code></pre>
  
  <p>What does the <code>&lt;update /&gt;</code> tag do?</p>