Document stale schema on 16.2 upgrade

What does this MR do and why?

See #427489 (closed) for more details, but this problem occurs only on no-downtime deployments.

Active Record caches the database schema in memory and it doesn't include the columns that have non-changed database values in the insert statements. The migrations are executed after the deployment and the schema is stale, needing an additional restart to refresh it.

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Related to #427489 (closed)