Clarify firing of import_notification_stream in doc comment

[ffarall]

Description

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

Closes Unexpected behaviour of block import_notification_stream.

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, and as discussed there, this PR aims to prevent this confusion in future devs looking to make use of this functionality.

Checklist

• My PR includes a detailed description as outlined in the "Description" and its two subsections above.
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• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

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