Add db_migrations job information

Why is this change being made?

Documentation is required to help describe the addition of an added run of a subset of QA specs to expand coverage of mixed environment type issues. This will assist in explaining what is happening within QA pipelines in deployments to the staging-canary environment as well as what additional steps by SETs may need to be taken during pipeline-triage rotations.

Related to gitlab-org/quality/quality-engineering/team-tasks#1881 (closed)

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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