Document DevTools' requirement for shutdown hook to be registered

Closes gh-4153

Author: wilkinsona

spring-boot-docs/src/main/asciidoc/using-spring-boot.adoc

@@ -886,6 +886,10 @@ NOTE: If you use JRebel automatic restarts will be disabled in favor of dynamic
 reloading. Other devtools features (such as LiveReload and property overrides) can still
 be used.
 
+NOTE: DevTools relies on the application context's shutdown hook to close it during a
+restart. It will not work correctly if you have disabled the shutdown hook (
+`SpringApplication.setRegisterShutdownHook(false)`).
+
 .Restart vs Reload
 ****
 The restart technology provided by Spring Boot works by using two classloaders.