More concrete `pair` documentation and example

[zzeroo]

This commit tries to clear the role of the pair parts.

Additional I've added an example and reformat the code with cargo fmt This commit was made in response to https://github.com/berkowski/tokio-serial/issues/25.

@SillyFreak thank you for the example. Sorry not working on all ci systems.

@berkowski I've tried to avoid the names master and slave because you've written as return type documentation.

Two connected, unnamed Serial objects.

If you prefer master/ slave I could change that.