nohz_full: Update based on Sedat Dilek review

Make it more clear that there are three options, and give hints as
to which of the three is most likely to be useful in different
situations.

Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>