cgroup: Update file naming comment

This changed long time ago in commit 8d7e6fb ("cgroup: update
cgroup name handling").

Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup-defs.h

@@ -619,9 +619,8 @@ struct cgroup_root {
  */
 struct cftype {
 	/*
-	 * By convention, the name should begin with the name of the
-	 * subsystem, followed by a period.  Zero length string indicates
-	 * end of cftype array.
+	 * Name of the subsystem is prepended in cgroup_file_name().
+	 * Zero length string indicates end of cftype array.
 	 */
 	char name[MAX_CFTYPE_NAME];
 	unsigned long private;