Sortable: Document the placeholder property in the ui argument of events. Fixes jquery#134.

Author: tjvantoll

includes/sortable-argument-ui.xml

@@ -17,4 +17,7 @@
 	<property name="sender" type="jQuery">
 		<desc>The sortable that the item comes from if moving from one sortable to another</desc>
 	</property>
+	<property name="placeholder" type="jQuery">
+		<desc>The jQuery object representing the element being used as a placeholder.</desc>
+	</property>
 </argument>