docs($q): note that function okToGreet is expected to be defined in example

the function okToGreet wasn't defined, so this example wouldn't work properly.

I've decided that instead of adding unrelated code to the example, it should just be noted that the
function is expected to be defined in the lexical scope.

Closes angular#5878

Author: letsmakesense

src/ng/q.js

@@ -16,9 +16,9 @@
  * asynchronous programming what `try`, `catch` and `throw` keywords are to synchronous programming.
  *
  * <pre>
- *   // for the purpose of this example let's assume that variables `$q` and `scope` are
- *   // available in the current lexical scope (they could have been injected or passed in).
- *
+ *   // for the purpose of this example let's assume that variables `$q`, `scope` and `okToGreet`
+ *   // are available in the current lexical scope (they could have been injected or passed in).
+ * 
  *   function asyncGreet(name) {
  *     var deferred = $q.defer();
  *