Polish Boot documentation

This removes the warnings produced by the invalid callout in the boot
reference. It also provides an example of how to customize the redis
connection and links to the "Connection to Redis" section of the
Spring Boot reference.

Issue #108

samples/boot/src/main/java/sample/config/HttpSessionConfig.java

@@ -1,11 +1,6 @@
 package sample.config;
 
-import org.springframework.context.annotation.Configuration;
-import org.springframework.session.data.redis.config.annotation.web.http.EnableRedisHttpSession;
+import org.springframework.session.data.redis.config.annotation.web.http.*;
 
-@Configuration
 @EnableRedisHttpSession // <1>
-public class HttpSessionConfig {
-
-
-}
+public class HttpSessionConfig { }