Document usage of devtools with build plugins

The `bootRun` gradle task or `spring-boot:run` maven goal can be used to
start a Spring Boot app with DevTools as long as forking is enabled. Add
an explicit note in the documentation to mention that.

Closes gh-3315

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

@@ -867,6 +867,10 @@ on the classpath change. This can be a useful feature when working in an IDE as
 a very fast feedback loop for code changes. By default, any entry on the classpath that
 points to a folder will be monitored for changes.
 
+NOTE: You can also start your application via the supported build plugins (i.e. Maven and
+Gradle) as long as forking is enabled since DevTools need an isolated application classloader
+to operate properly.
+
 TIP: Automatic restart works very well when used with LiveReload.
 <<using-boot-devtools-livereload,See below>> for details.