Improve the documentation of tokio-postgres connect

[JJaviMS]

As it can be seen on #924 and from my personal experience, it can be easily missed the use of the connection object when connecting using the tokio-postgres. I have improved the docs for that method so it indicates the importance of the connection object and give an example of how to use it.

[paolobarbolini]

Good idea. I've left some minor changes

[paolobarbolini]

I've cleaned it up a bit.

Suggested change

	/// This method returns the client which will allow you to interact with the database and a connection
	/// object which is the one that performs the communication with the dabase. **It's important to use the
	/// connection object as shown in the below example, if it is ignored then the client object will not work
	/// and when a .wait is called it will await forever.**
	/// This method returns the client object, which will allow you to interact with the database,
	/// and a connection object, which performs the communication with the database.
	///
	/// **The connection object must be actively polled, which is usually done by spawning it into
	/// it's own tokio task, as shown below. If this is not done, calls to the client object will hang forever.**

[paolobarbolini]

This should go below the examples section.

[paolobarbolini]

Suggested change

	/// # Examples
	/// ```
	/// // Connect to the database.
	/// let (client, connection) =
	/// tokio_postgres::connect("host=localhost user=postgres", NoTls).await?;
	/// // The connection object performs the actual communication with the database,
	/// // so spawn it off to run on its own.
	/// tokio::spawn(async move {
	/// if let Err(e) = connection.await {
	/// eprintln!("connection error: {}", e);
	/// }
	/// });
	/// // Use the client
	/// ```
	/// # Examples
	///
	/// ```
	/// // Connect to the database.
	/// let (client, connection) =
	/// tokio_postgres::connect("host=localhost user=postgres", NoTls).await?;
	///
	/// // The connection object performs the actual communication with the database,
	/// // so spawn it off to run on its own.
	/// tokio::spawn(async move {
	/// if let Err(e) = connection.await {
	/// eprintln!("connection error: {}", e);
	/// }
	/// });
	///
	/// // Now we can use the `client`
	/// ```