Clarify the Bootstrap docs notice with the dropdownParent option

This closes https://github.com/select2/select2/pull/4253.

docs/_includes/options/dropdown/placement.html

@@ -25,6 +25,10 @@ $('select').select2({
     I'm using a Bootstrap modal and I can'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>