docs: document the sysfs ABI for "isolated"

Add missing documentation of sysfs ABI for "isolated". It was added by
commit 59f30abe

("show isolated 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/20211201115957.254224-3-tarumizu.kohei@fujitsu.com


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