connection: clarify documented behavior of NotifyClose

[pabigot]

The documentation for NotifyClose() differs between Channel:

The chan provided will be closed when the Channel is closed and on a graceful close, no error will be sent.

and Connection:

On normal shutdowns, the chan will be closed.

The latter wording suggests that on abnormal shutdowns the chan will remain open, with two expected features:

• A non-blocking chan could be registered and the error notification would still be available to the application; and
• a new Connection could pass that same chan to NotifyClose() so an existing select statement need not update the chan it monitors.

In fact the logic for maintaining close notifiers appears identical for both Channel and Connection: both are closed as a result of the underlying object being closed.

This PR updates to use the Channel description in the Connection function to avoid the misleading interpretation.