update actor doc and add more actor based tests

tests/actor_start.ce

@@ -0,0 +1,13 @@
+// Test: $start lifecycle events (greet and stop)
+var got_greet = false
+$start(function(event) {
+  if (event.type == 'greet') {
+    if (!event.actor) disrupt
+    if (!is_actor(event.actor)) disrupt
+    got_greet = true
+  }
+  if (event.type == 'stop') {
+    if (!got_greet) disrupt
+    $stop()
+  }
+}, 'tests/actor_helper_stop')