ABI: sysfs-class-hwmon: add ABI documentation for it

Move the ABI attributes documentation from:
	Documentation/hwmon/sysfs-interface.rst

in order for it to follow the usual ABI documentation. That
allows script/get_abi.pl to properly handle it.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/5f47619ed882b0b8d1c84b56f7ea17bac0854b77.1632994837.git.mchehab+huawei@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

MAINTAINERS

@@ -8215,6 +8215,7 @@ L:	linux-hwmon@vger.kernel.org
 S:	Maintained
 W:	http://hwmon.wiki.kernel.org/
 T:	git git://git.kernel.org/pub/scm/linux/kernel/git/groeck/linux-staging.git
+F:	Documentation/ABI/testing/sysfs-class-hwmon
 F:	Documentation/devicetree/bindings/hwmon/
 F:	Documentation/hwmon/
 F:	drivers/hwmon/