Document devtools restart with Maven/Gradle

Update the documentation to help avoid confusion for people who aren't
using an IDE.

See gh-24271

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

@@ -711,6 +711,7 @@ As DevTools monitors classpath resources, the only way to trigger a restart is t
 The way in which you cause the classpath to be updated depends on the IDE that you are using.
 In Eclipse, saving a modified file causes the classpath to be updated and triggers a restart.
 In IntelliJ IDEA, building the project (`Build +->+ Build Project`) has the same effect.
+If using a build plugin, running `mvn compile` for Maven or `gradle build` for Gradle will trigger a restart. 
 ****
 
 NOTE: As long as forking is enabled, you can also start your application by using the supported build plugins (Maven and Gradle), since DevTools needs an isolated application classloader to operate properly.