iio:ABI docs: Combine sysfs-bus-iio-humidity-hdc2010/hdc100x into one file

These contain only one entry for out_current_heater_raw (_available).
Document this in a new sysfs-bus-iio-humidity file, and make it a little
more generic by allowing for non 0/1 values.

Fixes
$ scripts/get_abi.pl validate
Warning: /sys/bus/iio/devices/iio:deviceX/out_current_heater_raw is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-humidity-hdc2010:0  ./Documentation/ABI/testing/sysfs-bus-iio-humidity-hdc100x:0
Warning: /sys/bus/iio/devices/iio:deviceX/out_current_heater_raw_available is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-humidity-hdc2010:1  ./Documentation/ABI/testing/sysfs-bus-iio-humidity-hdc100x:1

Cc: Eugene Zaikonnikov <ez@norphonic.com>
Cc: Matt Ranostay <matt.ranostay@konsulko.com>
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Matt Ranostay <matt.ranostay@konsulko.com>
Link: https://lore.kernel.org/r/20210117153816.696693-6-jic23@kernel.org

Documentation/ABI/testing/sysfs-bus-iio-humidity-hdc2010 → Documentation/ABI/testing/sysfs-bus-iio-humidity

@@ -6,4 +6,5 @@ Description:
 		Controls the heater device within the humidity sensor to get
 		rid of excess condensation.
 
-		Valid control values are 0 = OFF, and 1 = ON.
+		In some devices, this is just a switch in which case 0 = OFF,
+		and 1 = ON.

Documentation/ABI/testing/sysfs-bus-iio-humidity-hdc100x

-What:		/sys/bus/iio/devices/iio:deviceX/out_current_heater_raw
-What:		/sys/bus/iio/devices/iio:deviceX/out_current_heater_raw_available
-KernelVersion:	4.3
-Contact:	linux-iio@vger.kernel.org
-Description:
-		Controls the heater device within the humidity sensor to get
-		rid of excess condensation.
-
-		Valid control values are 0 = OFF, and 1 = ON.