iio:ABI docs: Fix up duplicate *_calibbias_* documentation for icm42600

This device has the unusual characteristic that the calibbias values
have well defined units (more commonly they are tweaks to a DAC)
Unfortunately the previous approach of having more specific documentation
in sysfs-bus-iio-icm42600 results in warnings during the documentation
build and random ordering in the resulting documentation.

To avoid this, add a note to the main documentation on this special
characteristic for the icm42600.   The _available for calibbias was
missing from the main sysfs-bus-iio docs so also add that, allowing
us to drop the icm42600 specific file.

Fixes
$ scripts/get_abi.pl validate warning:
Warning: /sys/bus/iio/devices/iio:deviceX/in_accel_x_calibbias is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-icm42600:0  ./Documentation/ABI/testing/sysfs-bus-iio:394
Warning: /sys/bus/iio/devices/iio:deviceX/in_accel_y_calibbias is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-icm42600:1  ./Documentation/ABI/testing/sysfs-bus-iio:395
Warning: /sys/bus/iio/devices/iio:deviceX/in_accel_z_calibbias is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-icm42600:2  ./Documentation/ABI/testing/sysfs-bus-iio:396
Warning: /sys/bus/iio/devices/iio:deviceX/in_anglvel_x_calibbias is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-icm42600:3  ./Documentation/ABI/testing/sysfs-bus-iio:397
Warning: /sys/bus/iio/devices/iio:deviceX/in_anglvel_y_calibbias is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-icm42600:4  ./Documentation/ABI/testing/sysfs-bus-iio:398
Warning: /sys/bus/iio/devices/iio:deviceX/in_anglvel_z_calibbias is defined 2 times:  ./Documentation/ABI/testing/sysfs-bus-iio-icm42600:5  ./Documentation/ABI/testing/sysfs-bus-iio:399

Cc: Jean-Baptiste Maneyrol <jmaneyrol@invensense.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/20210117153816.696693-2-jic23@kernel.org