Add section for interpreting schema_inconsistencies_metric

What does this MR do?

The schema_inconsistencies_metric output is equivalent to the database schema checker Rake task. @alexives has made it clear that customers should not take these schema inconsistencies at face value, or even try to use them for troubleshooting without the guidance of Support.

A customer recently opened a ticket expressing concern about this output from the Service Ping: https://gitlab.zendesk.com/agent/tickets/590684 (internal).

This MR is intended to clarify that customers should not be worried about schema inconsistencies reported from the Service Ping, and that the metric is only designed for GitLab team members to troubleshoot ongoing issues.

Related issues

• #438916 (closed)
• #467544

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