update actor doc and add more actor based tests

tests/actor_requestors.ce

@@ -0,0 +1,30 @@
+// Test: sequence, parallel, and fallback requestor composition
+var immediate = function(callback, value) {
+  callback(42)
+}
+
+var add_one = function(callback, value) {
+  callback(value + 1)
+}
+
+var broken = function(callback, value) {
+  callback(null, "broken")
+}
+
+var _t = sequence([immediate, add_one])(function(result, reason) {
+  if (reason != null) disrupt
+  if (result != 43) disrupt
+
+  parallel([immediate, immediate])(function(results, reason) {
+    if (reason != null) disrupt
+    if (length(results) != 2) disrupt
+    if (results[0] != 42) disrupt
+    if (results[1] != 42) disrupt
+
+    fallback([broken, immediate])(function(result, reason) {
+      if (reason != null) disrupt
+      if (result != 42) disrupt
+      $stop()
+    })
+  })
+})