iio: improve doc for available_scan_mask

iio: improve doc for available_scan_mask

The available_scan_mask is an array of bitmaps representing the channels
which can be simultaneously enabled by the driver. In many cases, the
hardware can offer more channels than what the user is interested in
obtaining. In such cases, it may be preferred that only a subset of
channels are enabled, and the driver reads only a subset of the channels
from the hardware.

Some devices can't support all channel combinations. For example, the
BM1390 pressure sensor must always read the pressure data in order to
acknowledge the watermark IRQ, while reading temperature can be omitted.
So, the available scan masks would be 'pressure and temperature' and
'pressure only'.

When IIO searches for the scan mask it asks the driver to use, it will
pick the first suitable one from the 'available_scan_mask' array. Hence,
ordering the masks in the array makes a difference. We should 'prefer'
reading just the pressure from the hardware (as it is a cheaper operation
than reading both pressure and temperature) over reading both pressure
and temperature. Hence, we should set the 'only pressure' as the first
scan mask in available_scan_mask array. If we set the 'pressure and
temperature' as first in the array, then the 'only temperature' will never
get used as 'pressure and temperature' can always serve the user's
needs.

Add (minimal) kerneldoc to the 'available_scan_mask' to hint the user
that the ordering of masks matters.

Signed-off-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/4e43bf0186df5c8a56b470318b4827605f9cad6c.1695727471.git.mazziesaccount@gmail.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>