Add comment for photo_card_toolbutton.

frontends/etiquette_flask/templates/cards.html

@@ -161,6 +161,7 @@ draggable="true"
 
     <span class="photo_card_tags" title="{{tag_names_title}}">{{tag_names_inner}}</span>
 
+    {# This button should be unhidden by any page that inserts toolbox items #}
     <button class="photo_card_toolbutton hidden" onclick="return cards.photos.show_tools(event);"></button>
     <div class="photo_card_tools" onclick="event.stopPropagation(); return;">
     </div>