Fix some comments in the examples. (#187)

The PR that updated the examples skipped some comments. This patch
updates thhe comments.

examples/hello_world.rs

@@ -54,22 +54,15 @@ pub fn main() {
 
     println!("server running on localhost:6142");
 
-    // This starts the `current_thread` executor.
+    // Start the Tokio runtime.
     //
-    // Executors are responsible for scheduling many asynchronous tasks, driving
-    // them to completion. There are a number of different executor
-    // implementations, each providing different scheduling characteristics.
+    // The Tokio is a pre-configured "out of the box" runtime for building
+    // asynchronous applications. It includes both a reactor and a task
+    // scheduler. This means applications are multithreaded by default.
     //
-    // The `current_thread` executor multiplexes all scheduled tasks on the
-    // current thread. This means that spawned tasks must not implement `Send`.
-    // It's important to note that all futures / tasks are lazy. No work will
-    // happen unless they are spawned onto an executor.
-    //
-    // The executor will start running the `server` task, which, in turn, spawns
-    // new tasks for each incoming connection.
-    //
-    // The `current_thread::block_on_all` function will block until *all*
-    // spawned tasks complete.
+    // This function blocks until the runtime reaches an idle state. Idle is
+    // defined as all spawned tasks have completed and all I/O resources (TCP
+    // sockets in our case) have been dropped.
     //
     // In our example, we have not defined a shutdown strategy, so this will
     // block until `ctrl-c` is pressed at the terminal.