Clarify the Bootstrap docs notice with the dropdownParent option

This closes select2#4253.

Author: kevin-brown

docs/_includes/options/dropdown/placement.html

@@ -25,6 +25,10 @@ <h3>
     I&apos;m using a Bootstrap modal and I can&apos;t use the search box
   </h3>
 
+  <p>
+    Use the <code>dropdownParent</code> option, setting it to the modal.
+  </p>
+
   {% include options/not-written.html %}
 
   <h3>