docs(migration): add a note about "private" properties being reverted

[rodyhaddad]

Just looked at the migration guide and this caught my attention :)

If there's a better way to formulate it, or it shouldn't be an alert-info, feel free to suggest an alternative and I'll do the changes if necessary.

[mary-poppins]

Thanks for the PR!

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Thanks again for your help!

[rodyhaddad]

CLA signed as Rodric Haddad