docs: document the sysfs ABI for "nohz_full"

Add missing documentation of sysfs ABI for "nohz_full". It was added
by commit 6570a9a1("show nohz_full cpus in sysfs"). However,
there is no documentation for these interface.

Signed-off-by: Kohei Tarumizu <tarumizu.kohei@fujitsu.com>
Link: https://lore.kernel.org/r/20211201115957.254224-2-tarumizu.kohei@fujitsu.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>