title: Woocommerce backorder notification in checkout page
body: <p>I am trying to add an action that will check whether or not a product that is currently only available on back order is being checked out - and in the case that <strong>one or more is</strong>, I want to display a message before the checkout form.</p>
  
  <p>I've gotten this far:</p>
  
  <pre><code>add_action( 'woocommerce_before_checkout_form', 'wnd_checkout_message', 10 );
  function wnd_checkout_message( ) {
  echo '&lt;div class="wnd-checkout-message"&gt;&lt;h3&gt;The message goes here!&lt;/h3&gt;&lt;/div&gt;';}
  </code></pre>
  
  <p>But how do I check whether or not a back ordered product is currently being checked out/is in the cart?</p>