jQuery.widget: Document that instance can be called on any element

Fixes gh-77

Author: scottgonzalez

includes/widget-method-instance.xml

@@ -1,6 +1,8 @@
 <?xml version="1.0"?>
-<method name="instance">
+<method name="instance" return="Object">
 	<desc>
-		Retrieves the <placeholder name="name"/>'s instance object.
+		<p>Retrieves the <placeholder name="name"/>'s instance object. If the element does not have an associated instance, <code>undefined</code> is returned.</p>
+
+		<p>Unlike other widget methods, <code>instance()</code> is safe to call on any element after the <placeholder name="name"/> plugin has loaded.</p>
 	</desc>
 </method>