cgroup: Make Kconfig prompt of debug cgroup more accurate

The Kconfig prompt and description of the debug cgroup controller
more accurate by saying that it is for debug purpose only and its
interfaces are unstable.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

init/Kconfig

@@ -1205,11 +1205,14 @@ config CGROUP_BPF
 	  inet sockets.
 
 config CGROUP_DEBUG
-	bool "Example controller"
+	bool "Debug controller"
 	default n
+	depends on DEBUG_KERNEL
 	help
 	  This option enables a simple controller that exports
-	  debugging information about the cgroups framework.
+	  debugging information about the cgroups framework. This
+	  controller is for control cgroup debugging only. Its
+	  interfaces are not stable.
 
 	  Say N.