User Password Expiry page and the description of disconnect_on_expired_password variable describe it's behavior as "disconnecting" the client, which reads like it affects existing connections. In fact, it only affects the result of connection attempt – it either prevents the connection, or allows it. While MySQL manual uses similar terms, it's more understandable, since it describes the behavior more verbosely.

Also, the User Password Expiry page should explain the use of --connect-expired-password client option.