stm32/dac: Follow ref-man guidelines on GPIO config

GPIO pins should be configured for Analog IN on virtually all families
to reduce parasitic consumption.

Reported-by: Roger Wolff <R.E.Wolff@BitWizard.nl>

lib/stm32/common/dac_common_all.c

@@ -11,8 +11,11 @@ of the connection line, high density and XL devices.
 Two DAC channels are available, however unlike the ADC channels these
 are separate DAC devices controlled by the same register block.
 
-The DAC is on APB1. Its clock must be enabled in RCC and the GPIO
+The DAC is on APB1. Its clock must be enabled in RCC and depending on
+specific family, the GPIO
 ports set to alternate function output before it can be used.
+On most families, the GPIO pins should be configured to Analog IN to
+avoid parasitic consumption.
 The digital output driver is disabled so the output driver mode
 (push-pull/open drain) is arbitrary.
 
@@ -48,7 +51,7 @@ by specifying the appropriate register to the DMA controller.
 
 @section dac_api_basic_ex Basic DAC handling API.
 
-Set the DAC's GPIO port to any alternate function output mode. Enable the
+Set the DAC's GPIO port to Analog IN. Enable the
 DAC clock. Enable the DAC, set a trigger source and load the buffer
 with the first value. After the DAC is triggered, load the buffer with
 the next value. This example uses software triggering and added noise.