added some doc comments

units/digital_in/unit_din.c

@@ -173,6 +173,7 @@ static void DI_deInit(Unit *unit)
 
 // ------------------------------------------------------------------------
 
+/** Read request */
 error_t UU_DI_Read(Unit *unit, uint16_t *packed)
 {
     CHECK_TYPE(unit, &UNIT_DIN);

units/digital_in/unit_din.h

@@ -9,4 +9,13 @@
 
 extern const UnitDriver UNIT_DIN;
 
+/**
+ * Read pins
+ *
+ * @param unit
+ * @param packed - output; the packed (right aligned) bits representing the pins, highest to lowest, are written here.
+ * @return success
+ */
+error_t UU_DI_Read(Unit *unit, uint16_t *packed);
+
 #endif //U_DIN_H