Documentation: update cgroups info user groups names

Update suitable words to explain / understand cgroups contents.

Signed-off-by: Geunsik Lim <geunsik.lim@samsung.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>