Add docs on deduplication of idempotent workers

What does this MR do?

This adds a note on when we deduplicate idempotent jobs from the sidekiq queues.

Related issues

gitlab-com/gl-infra/scalability#298 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• [-] If applicable, update the permissions table.
• [-] Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.

Review checklist

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3. Maintainer

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