INT-3295 Add Doc For Global MessageConverter

JIRA: https://jira.springsource.org/browse/INT-3295

Plus test.

spring-integration-core/src/test/java/org/springframework/integration/channel/config/ChannelParserTests.java

@@ -166,6 +166,15 @@ public class ChannelParserTests {
 		assertTrue(TestUtils.getPropertyValue(channel, "messageConverter") instanceof UselessMessageConverter);
 	}
 
+	@Test
+	public void testDatatypeChannelGlobalConverter() {
+		ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("channelParserGlobalConverterTests.xml", this
+				.getClass());
+		MessageChannel channel = context.getBean("integerChannel", MessageChannel.class);
+		context.close();
+		assertTrue(TestUtils.getPropertyValue(channel, "messageConverter") instanceof UselessMessageConverter);
+	}
+
 	@Test
 	public void testDatatypeChannelWithAssignableSubTypes() {
 		ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext("channelParserTests.xml", this

spring-integration-core/src/test/java/org/springframework/integration/channel/config/channelParserGlobalConverterTests.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<beans:beans xmlns="http://www.springframework.org/schema/integration"
+	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:beans="http://www.springframework.org/schema/beans"
+	xsi:schemaLocation="http://www.springframework.org/schema/beans
+			http://www.springframework.org/schema/beans/spring-beans.xsd
+			http://www.springframework.org/schema/integration
+			http://www.springframework.org/schema/integration/spring-integration.xsd">
+
+	<channel id="integerChannel" datatype="java.lang.Integer">
+		<queue capacity="10" />
+	</channel>
+
+	<beans:bean id="datatypeChannelMessageConverter"
+		class="org.springframework.integration.channel.config.ChannelParserTests$UselessMessageConverter" />
+
+</beans:beans>

src/reference/docbook/channel.xml

@@ -521,6 +521,11 @@ payload to an Integer.
             payload, or a full <classname>Message</classname> object. If the latter, the converter must
             be careful to copy all the headers from the inbound message.
         </para>
+        <para>
+            Alternatively, declare a <code>&lt;bean/&gt;</code> of type <interfacename>MessageConverter</interfacename>
+            with an id <code>"datatypeChannelMessageConverter"</code> and that converter will be used by
+            all channels with a <code>datatype</code>.
+        </para>
     </section>
 
     <section id="channel-configuration-queuechannel">