Restore docs: Warn about DB overwrite more prominently

What does this MR do?

Highlights the existing warnings about a restore overwriting the DB, and introduces 2 more in the sections used for entry into the restore procedure.

Related issues

• Further context in Slack

[based on existing docs …]

• Restoring is required to back out database data and schema changes (migrations) made as part of the upgrade.
• GitLab backups must be restored to the exact same version and edition.

… it seems to be the logical conclusion of not being able to use a backup created directly before the downgrade, but our docs don't really warn against downgrades that are performed much later than the [initial] upgrade.

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