Clarify firing of `import_notification_stream` in doc comment (#5811)

# Description

Updates the doc comment on the `import_notification_stream` to make its
behaviour clearer.

Closes [Unexpected behaviour of block
`import_notification_stream`](https://github.com/paritytech/polkadot-sdk/issues/5596).

## Integration

Doesn't apply.

## Review Notes

The old comment docs caused some confusion to myself and some members of
my team, on when this notification stream is triggered. This is
reflected in the linked
[issue](https://github.com/paritytech/polkadot-sdk/issues/5596), and as
discussed there, this PR aims to prevent this confusion in future devs
looking to make use of this functionality.

# Checklist

* [x] My PR includes a detailed description as outlined in the
"Description" and its two subsections above.
* [ ] My PR follows the [labeling requirements](

https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md#Process
) of this project (at minimum one label for `T` required)
* External contributors: ask maintainers to put the right label on your
PR.
* [x] I have made corresponding changes to the documentation (if
applicable)
* [x] I have added tests that prove my fix is effective or that my
feature works (if applicable)

You can remove the "Checklist" section once all have been checked. Thank
you for your contribution!

---------

Co-authored-by: Michal Kucharczyk <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>