Documentation: update cgroup pid and cpuset information

The cgroup documentation does not specify how a process can be removed
from a particular group.  This patch adds a note at the end of the
simple example about how this is done.  Also, some cgroups (like
cpusets) require user input before a new group can be used.  This is
noted in the patch as well.

Signed-off-by: Eric B Munson <emunson@mgebm.net>
Acked-by: Paul Menage <menage@google.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>