iio: Documentation: update definitions for bufferY and scan_elements

iio: Documentation: update definitions for bufferY and scan_elements

Since the new change to the IIO buffer infrastructure, the buffer/ and
scan_elements/ directories have been merged into bufferY/ to have some
attributes available per-buffer.

This change updates the ABI docs to reflect this change.

The hwfifo attributes are not updated, as for now these should be used
via the legacy buffer/ directory until they are moved into core.

Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Link: https://lore.kernel.org/r/20210217083438.37865-2-alexandru.ardelean@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>