Update hotplug cpu documentation to clearly state when to use
register_cpu_notifier() and register_hotcpu_notifier.

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>