Output of `python -m websockets` changed, breaking the documentation.

[aaugustin]

python -m websockets ... used to display Connection closed: 1000 (OK). or Connection closed: 1001 (going away).

This appears in several places in the documentation. Specifically, it's used in the deployment how-to to demonstrate proper shutdown (which no one cares very much about, apparently, as no one reported it.)

Since ce06dd6 it displays only Connection closed.. I believe that I made that change on purpose because no one cares very much about close codes either :-)

Options:

1. Restore the previous output.
2. Update the documentation & provide a new way to check how the connection is closed.