iio: buffer: document iio_push_to_buffers_with_ts_unaligned() may sleep

iio: buffer: document iio_push_to_buffers_with_ts_unaligned() may sleep

Add Context: documentation comment that
iio_push_to_buffers_with_ts_unaligned() may sleep because it calls
devm_krealloc().

Also document Return: value while here.

Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: David Lechner <dlechner@baylibre.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>