Adding note about imported objects showing old timestamps and new author

What does this MR do and why?

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A customer came across this when a freshly onboarded developer imported one of our sample projects. The MRs and issues included in this sample all showed in the newly created project as having been created by the freshly created user long before they had a GitLab account.

I think we should highlight that this is expected behavior as this can lead to confusion.

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