update actor doc and add more actor based tests

tests/actor_connection.ce

@@ -0,0 +1,9 @@
+// Test: $connection reports local for a child actor
+$start(function(event) {
+  if (event.type == 'greet') {
+    $connection(function(info) {
+      if (info.type != "local") disrupt
+      $stop()
+    }, event.actor, {})
+  }
+}, 'tests/actor_helper_echo')