iio:adc:Kconfig: rework help descriptions

Rework the help text of several ADCs to make sure that:
  - the module name is mentioned, if the driver can be built as a module
  - "If unsure, say N" messages are dropped
  - right indentation is maintained

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>