Documentation: iio: Add ADC documentation

ADC inputs can be classified into a few different types according to how
they measure the input signal, how restrained the signal is, and number of
input pins. Even though datasheets tend to provide many details about their
inputs and measurement procedures, it may not always be clear how to model
those inputs into IIO channels.

For example, some differential ADCs can have their inputs configured into
pseudo-differential channels. In that configuration, only one input
connects to the signal of interest as opposed to using two inputs of a
differential input configuration. Datasheets sometimes also refer to
pseudo-differential inputs as single-ended inputs even though they have
distinct physical configuration and measurement procedure.

Document consolidated ADC input types and how they are usually described
and supported in device tree and IIO, respectively.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Reviewed-by: David Lechner <dlechner@baylibre.com>
Link: https://patch.msgid.link/e6ac2a595f06ba2d5ff0eb86e5895479c9dd797f.1739998491.git.marcelo.schmitt@analog.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

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