doc: Is it safe to continue to write to mongodb after upgrade with migrations running in the background?

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Consider an Upgrade from 2.x to 5.4.0, with xapi-service and learning-locker both deployed and pointing to the database. When applying the outstanding database migrations using the CLI command, is it safe to continue to use xapi-service to insert new statements while the migration(s) are still running, or is it considered unsafe? Do new documents get inserted into the migration stream?