Clarify how to exclude devtools with war packaging.

Closes gh-7556

spring-boot-tools/spring-boot-maven-plugin/src/site/apt/usage.apt.vm

@@ -60,6 +60,11 @@ Usage
   Please note that the <<<outputFileNameMapping>>> feature of the <<<maven-war-plugin>>>
   is currently not supported.
 
+  It is also possible to automatically exclude devtools using the <<<excludeDevtools>>>
+  property. In order to make that work with <<<war>>> packaging, the
+  `spring-boot-devtools` dependency must be set as <<<optional>>> or with the
+  <<<provided>>> scope.
+
   The original (i.e. non executable) artifact is renamed to <<<.original>>> by default but it is also
   possible to keep the original artifact using a custom classifier.