Add Service Desk Incoming email clarification

What does this MR do?

It's not clear when reading the Service Desk setup instructions that Service Desk will use the Incoming email configuration by default, and that creating a separate Service Desk mailbox configuration is only necessary if you are using a custom mailbox.

This came up in the course of working a customer ticket (Internal ZenDesk Link) and becoming confused about the working configuration and what configuration settings were necessary.

Related issues

Author's checklist

• Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
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If you are only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Review checklist

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• 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.