Correct documentation for protocol version

Greetings,

The current docs say that if a client asks for a protocol that the backend doesn't support, it will return the newest minor version. https://www.postgresql.org/docs/current/protocol-message-formats.html#PROTOCOL-MESSAGE-FORMATS-NEGOTIATEPROTOCOLVERSION

However that isn't what it returns. It actually returns the entire newest protocol that it supports. Attached is a patch to fix the docs.