iio: documentation: Add out_altvoltage and friends

Continuous frequency/clock generating devices, such as DDSs or PLLs
should use out_altvoltage.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>