cgroups: document the effect of attaching PID 0 to a cgroup

Document that a pid of zero(0) can be used to refer to the current task
when attaching a task to a cgroup, as in the following usage:

	# echo 0 > /dev/cgroup/tasks

This is consistent with existing cpuset behavior.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Paul Jackson <pj@sgi.com>
Acked-by: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Andrea Righi <righi.andrea@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>