iio:ABI docs: Move specific description of out_altvoltageX_frequency to main docs.

The adf4371 has channels that are very closely coupled, so additional
documentation is needed to express these constraints.
Unfortunately having the same sysfs filename in multiple documentation
does not work well when generating automated documentation.
To avoid this issue, we add a new device specific description to the
main docs and remove the one in the device specific file.

Fixes
$ scripts/get_abi.pl validate
Warning: /sys/bus/iio/devices/iio:deviceX/out_altvoltageY_frequency is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-frequency-adf4371:0  ./Documentation/ABI/testing/sysfs-bus-iio:599

Cc: Alexandru Ardelean <alexandru.ardelean@analog.com>
Reported-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20210117153816.696693-8-jic23@kernel.org