periph/adc: add doc on low power implications

drivers/include/periph/adc.h

@@ -30,6 +30,18 @@
  * waiting for the result of a conversion (e.g. through putting the calling
  * thread to sleep while waiting for the conversion results).
  *
+ * # (Low-) Power Implications
+ *
+ * The ADC peripheral(s) **should** only be powered on while adc_sample() is
+ * active. For implementing adc_sample() this means, that the peripheral should
+ * be powered on (i.e. through peripheral clock gating) at the beginning of the
+ * function and it should be powered back off at the end of the function.
+ *
+ * If the adc_sample() function is implemented in a way, that it will put the
+ * active thread to sleep for a certain amount of time, the implementation
+ * might need to block certain power states.
+ *
+ *
  * @todo        Extend interface for continuous mode?
  *
  * @{