[GitLab Dedicated] Add notes on log identification

Why is this change being made?

This change is being made as part of our effort to bring the Opensearch log information for GitLab Dedicated to parity with the information available for searching through Kibana.

See gitlab-com/gl-infra/gitlab-dedicated/team#2572 for more information on this effort and other related MRs.

In particular, this suggests that Support Engineers use a self-managed test instance to generate the log entries that they will be searching for in Opensearch.

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