Driver documentation update

libuavcan/include/uavcan/driver/system_clock.hpp

@@ -23,23 +23,34 @@ public:
 
     /**
      * Monototic system clock.
-     * This shall never jump during UTC timestamp adjustments; the base time is irrelevant.
+     *
+     * This clock shall never jump or change rate; the base time is irrelevant.
+     * This clock is mandatory and must remain functional at all times.
+     *
      * On POSIX systems use clock_gettime() with CLOCK_MONOTONIC.
      */
     virtual MonotonicTime getMonotonic() const = 0;
 
     /**
-     * UTC clock.
-     * This can jump when the UTC timestamp is being adjusted.
-     * Return 0 if the UTC time is not available yet (e.g. the device just started up with no battery clock).
-     * On POSIX systems use gettimeofday().
+     * Global network clock.
+     * It doesn't have to be UTC, the name is a bit misleading - actual time base doesn't matter.
+     *
+     * This clock can be synchronized with other nodes on the bus, hence it can jump and/or change
+     * rate occasionally.
+     * This clock is optional; if it is not supported, return zero. Also return zero if the UTC time
+     * is not available yet (e.g. the device has just started up with no battery clock).
+     *
+     * For POSIX refer to clock_gettime(), gettimeofday().
      */
     virtual UtcTime getUtc() const = 0;
 
     /**
-     * Set the UTC system clock.
-     * @param [in] adjustment Amount of time to add to the clock value.
+     * Adjust the network-synchronized clock.
+     * Refer to @ref getUtc() for details.
+     *
      * For POSIX refer to adjtime(), settimeofday().
+     *
+     * @param [in] adjustment Amount of time to add to the clock value.
      */
     virtual void adjustUtc(UtcDuration adjustment) = 0;
 };