Selectmenu: Add documentation for _renderButtonItem() extension point

Closes gh-226

entries/selectmenu.xml

@@ -203,6 +203,46 @@ _renderMenu: function( ul, items ) {
 _resizeMenu: function() {
 	this.menu.outerWidth( 500 );
 }
+]]></code>
+			</example>
+		</method>
+		<method name="_renderButtonItem" return="jQuery">
+			<desc>
+				<p>Method that controls the creation of the generated button content. The method must create and return a new element.</p>
+			</desc>
+			<argument name="item" type="Object">
+				<property name="disabled" type="Boolean">
+					<desc>Whether the item is disabled.</desc>
+				</property>
+				<property name="element" type="jQuery">
+					<desc>A reference to the item's original <code>&lt;option&gt;</code> element.</desc>
+				</property>
+				<property name="index" type="Number">
+					<desc>The numeric index of the item.</desc>
+				</property>
+				<property name="label" type="String">
+					<desc>The string to display for the item.</desc>
+				</property>
+				<property name="optgroup" type="String">
+					<desc>If the item is within an <code>&lt;optgroup&gt;</code>, this is set to that <code>&lt;optgroup&gt;</code>'s label.</desc>
+				</property>
+				<property name="value" type="String">
+					<desc>The <code>value</code> attribute of the item's original <code>&lt;option&gt;</code>.</desc>
+				</property>
+			</argument>
+			<example>
+				<desc>Style the button background color based on the value of the selected option.</desc>
+				<code><![CDATA[
+_renderButtonItem: function( item ) {
+	var buttonItem = $( "<span>", {
+		"class": "ui-selectmenu-text"
+	})
+	this._setText( buttonItem, item.label );
+
+	buttonItem.css( "background-color", item.value )
+
+	return buttonItem;
+}
 ]]></code>
 			</example>
 		</method>