cpuset: various documentation fixes and updates

I noticed the old commit 8f5aa26c


("cpusets: update_cpumask documentation fix") is not a complete fix,
resulting in inconsistent paragraphs.  This patch fixes it and does other
fixes and updates:

- s/migrate_all_tasks()/migrate_live_tasks()/
- describe more cpuset control files
- s/cpumask_t/struct cpumask/
- document cpu hotplug and change of 'sched_relax_domain_level' may cause
  domain rebuild
- document various ways to query and modify cpusets
- the equivalent of "mount -t cpuset" is "mount -t cgroup -o cpuset,noprefix"

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Paul Menage <menage@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>