Document workers that need their own queue

What does this MR do?

Some Sidekiq workers cannot share a queue with other Sidekiq workers, and must continue to have their own named queue. We can't fix them all at once, but we can document this and link to the correct issues.

Also, fix the urgency of ServiceDeskEmailReceiverWorker.

Related issues

For gitlab-com/gl-infra/scalability#1087 (closed).

Author's checklist

• Follow the:
  • Documentation process.
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  • Style Guide.
• Request a review based on:
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Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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• Ensure a release milestone is set.