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

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -666,3 +666,10 @@ Description:	Preferred MTE tag checking mode
 		================  ==============================================
 
 		See also: Documentation/arm64/memory-tagging-extension.rst
+
+What:		/sys/devices/system/cpu/nohz_full
+Date:		Apr 2015
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:
+		(RO) the list of CPUs that are in nohz_full mode.
+		These CPUs are set by boot parameter "nohz_full=".