Improve documentation of matrix variables.

Prior to this commit, it was not clear how to enable the support of matrix
variables in the mvc namespace. As the feature is disabled by default, added
something to highlight the part that explains how to configure it

Issue: SPR-11331

Author: snicoll

src/asciidoc/index.adoc

@@ -28780,6 +28780,38 @@ Note that to enable the use of matrix variables, you must set the
 `removeSemicolonContent` property of `RequestMappingHandlerMapping` to `false`. By
 default it is set to `true`.
 
+[TIP]
+====
+
+The MVC Java config and the MVC namespace both provide options for enabling the use of
+matrix variables.
+
+If you are using Java config, The <<mvc-config-advanced-java, Advanced Customizations
+with MVC Java Config>> section describes how the `RequestMappingHandlerMapping` can
+be customized.
+
+In the MVC namespace, the `<mvc:annotation-driven>` element has an
+`enableMatrixVariables` attribute that should be set to `true`. By default it is set
+to `false`.
+
+[source,xml,indent=0]
+[subs="verbatim,quotes"]
+----
+	<?xml version="1.0" encoding="UTF-8"?>
+	<beans xmlns="http://www.springframework.org/schema/beans"
+		xmlns:mvc="http://www.springframework.org/schema/mvc"
+		xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+		xsi:schemaLocation="
+			http://www.springframework.org/schema/beans
+			http://www.springframework.org/schema/beans/spring-beans.xsd
+			http://www.springframework.org/schema/mvc
+			http://www.springframework.org/schema/mvc/spring-mvc.xsd">
+
+		<mvc:annotation-driven enableMatrixVariables="true"/>
+
+	</beans>
+----
+====
 
 [[mvc-ann-requestmapping-consumes]]
 ===== Consumable Media Types