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