scheduler: (runcount) Exlain absence of atomic-box-compare-and-swap!.

Closes: #48
Reported-by: cireu (with some adjustments)

* fibers/scheduler.scm (run-scheduler)[run-task]: Explain it.

Author: emixa-d

fibers/scheduler.scm

@@ -284,6 +284,8 @@ value.  Return zero values."
         (cur (scheduler-current-runqueue sched))
         (steal-work! (work-stealer sched)))
     (define (run-task task)
+      ;; The only thread writing to the runcount box is the thread of SCHED,
+      ;; so no need for atomic-box-compare-and-swap! here.
       (atomic-box-set! runcount-box
                        (logand (1+ (atomic-box-ref runcount-box)) #xffffFFFF))
       (call-with-prompt tag