Fixes #13835: More explicitly document mobile dropdown menu dismissing behavior

docs/_includes/js/dropdowns.html

@@ -96,7 +96,9 @@
 
 
   <h2 id="dropdowns-usage">Usage</h2>
-  <p>Via data attributes or JavaScript, the dropdown plugin toggles hidden content (dropdown menus) by toggling the <code>.open</code> class on the parent list item. When opened, the plugin also adds <code>.dropdown-backdrop</code> as a click area for closing dropdown menus when clicking outside the menu. Note: The <code>data-toggle=dropdown</code> attribute is relied on for closing dropdown menus at an application level, so it's a good idea to always use it.</p>
+  <p>Via data attributes or JavaScript, the dropdown plugin toggles hidden content (dropdown menus) by toggling the <code>.open</code> class on the parent list item.</p>
+  <p>On mobile devices, opening a dropdown adds a <code>.dropdown-backdrop</code> as a tap area for closing dropdown menus when tapping outside the menu, a requirement for proper iOS support. <strong class="text-danger">This means switching to another dropdown menu requires an extra tap on mobile.</strong></p>
+  <p>Note: The <code>data-toggle=dropdown</code> attribute is relied on for closing dropdown menus at an application level, so it's a good idea to always use it.</p>
 
   <h3>Via data attributes</h3>
   <p>Add <code>data-toggle="dropdown"</code> to a link or button to toggle a dropdown.</p>