iio: ABI: use Y consistently as channel number

Change X to Y when referring to channel number in the ABI documentation.
There were only a few cases using X (and one using Z). By far, most
documented attributes are using Y for the channel number placeholder.
For consistency, we should follow the same convention throughout.

Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/20241216-doc-iio-use-voltagey-consistently-v1-1-9e34a72133bc@baylibre.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>