clockevent: document cpu locality

Each cpu has its own event timer; we rely on this.  Document the requirement.

drivers/clockevent.hh

@@ -6,12 +6,14 @@
 class clock_event_callback {
 public:
     virtual ~clock_event_callback();
+    // note: must always be called on the same cpu that the timer was set on
     virtual void fired() = 0;
 };
 
 class clock_event_driver {
 public:
     virtual ~clock_event_driver();
+    // set() is cpu-local: each processor has its own timer
     virtual void set(u64 time) = 0;
     void set_callback(clock_event_callback* callback);
     clock_event_callback* callback() const;