change flush() comments on example code

- remove flush() call from complete example code
- change phrasing on publish-only example comment

async-nats/src/lib.rs

@@ -54,9 +54,6 @@
 //! client.publish("messages", "data".into()).await?;
 //! }
 //!
-//! // Flush internal buffer to make sure messages are sent
-//! client.flush().await?;
-//!
 //! // Receive and process messages
 //! while let Some(message) = subscriber.next().await {
 //! println!("Received message {:?}", message);
@@ -88,7 +85,7 @@
 //! client.publish(subject, data.clone()).await?;
 //! }
 //!
-//! // Flush internal buffer to make sure messages are sent
+//! // Flush internal buffer before exiting to make sure all messages are sent
 //! client.flush().await?;
 //!
 //! # Ok(())