Document what features are broken when db_key_base is lost

What does this MR do?

GitLab recently went on a drive to encrypt more database columns with db_key_base. This is good, but it does mean that the effects of losing that key are more wide-ranging than they used to be.

Add some documentation that covers the features that are broken when this key is lost, and correct a few minor things about the existing "how to delete the useless encrypted data" example.

What are the relevant issue numbers?

Closes #57690 (closed)

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Conforms to the code review guidelines
• Conforms to the style guides