Add text "singleton" to header of singleton classes.

Because the icon isn't explained anywhere, it might not be immediately apparent
that a class is singleton.

template/app/view/cls/Header.js

@@ -19,6 +19,9 @@ Ext.define('Docs.view.cls.Header', {
                 '<a href="#" class="class-source-link">{name}',
                   '<span class="class-source-tip">View source...</span>',
                 '</a>',
+                '<tpl if="singleton">',
+                    '<span>singleton</span>',
+                '</tpl>',
                 '{[this.renderAliases(values.aliases)]}',
             '</h1>',
             Docs.data.showPrintButton ? '<a class="print" href="?print=/api/{name}" target="_blank">Print</a>' : '',