Document how to check background migrations before attempting an upgrade

Summary

In #4501, users are running into an issue with upgrades because migrations are still running in the background.

Solution

As an interim solution to help prevent ongoing issues when upgrading, we should add documentation to the upgrade docs on how to check if background migrations have completed. In a future release we should put guardrails in place. This documentation issue is expected to uncover details that will inform the solution for adding guardrails to prevent upgrades when background migrations are still running.