revert documentaion update for memcg's dirty ratio.

Subjct: Revert memory cgroup dirty_ratio Documentation.

The commit ece72400 adds documentation
for memcg's dirty ratio. But the function is not implemented yet.
Remove the documentation for avoiding confusing users.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Greg Thelen <gthelen@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>