Add some comments to or_circuit_t.workqueue_entry

These were suggested by dgoulet, thanks!

src/or/cpuworker.c

@@ -556,6 +556,7 @@ cpuworker_cancel_circ_handshake(or_circuit_t *circ)
     tor_free(job);
     tor_assert(total_pending_tasks > 0);
     --total_pending_tasks;
+    /* if (!job), this is done in cpuworker_onion_handshake_replyfn. */
     circ->workqueue_entry = NULL;
   }
 }

src/or/or.h

@@ -3126,7 +3126,8 @@ typedef struct or_circuit_t {
    * chance to give an onionskin to a cpuworker. Used only in onion.c */
   struct onion_queue_t *onionqueue_entry;
   /** Pointer to a workqueue entry, if this circuit has given an onionskin to
-   * a cpuworker and is waiting for a response. Used only in cpuworker.c */
+   * a cpuworker and is waiting for a response. Used to decide whether it is
+   * safe to free a circuit or if it is still in use by a cpuworker. */
   struct workqueue_entry_s *workqueue_entry;
 
   /** The circuit_id used in the previous (backward) hop of this circuit. */