Clarify email address for auditor user signup

Why is this change being made?

Initial description of setting up Auditor account can result in misunderstanding, causing the Auditor account to not have a valid email address. This has happened to some new Support Engineers while reading the instruction literally.

Although this is further clarified further down in the section, the change just makes the entire section consistent and easier to understand at a glance, and may prevent this happening to other new Support Engineers.

Author Checklist

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• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
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