add function comment

svn:r4043

src/or/cpuworker.c

@@ -358,6 +358,10 @@ static void process_pending_task(connection_t *cpuworker) {
 
 #define CPUWORKER_BUSY_TIMEOUT 100 /* seconds */
 
+/** We have a bug that I can't find. Sometimes, very rarely, cpuworkers
+ * get stuck in the 'busy' state, even though the cpuworker process
+ * thinks of itself as idle. I don't know why. But here's a workaround
+ * to kill any cpuworker that's been busy for more than 100 seconds. */
 static void
 cull_wedged_cpuworkers(void) {
   connection_t **carray;