Add multiple selection specification to docs

docs/_includes/examples/tags.html

@@ -2,7 +2,8 @@
   <h1 id="tags">Tagging support</h1>
 
   <p>
-    Select2 can be used to quickly set up fields used for tagging.
+    Select2 can be used to quickly set up fields used for tagging. 
+    <code>multiple</code> have to be enabled on the field for tags support.
   </p>
 
   <p>