doc: Updates the Migration guide for the new User Management resources. (#3681)

* docs: Format doc file.

* minor doc changes.

* Update docs/guides/atlas-user-management.md

Co-authored-by: Copilot <[email protected]>

* Apply suggestions from code review

Co-authored-by: John Williams <[email protected]>

* address comments.

* address comments.

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Co-authored-by: Copilot <[email protected]>
Co-authored-by: John Williams <[email protected]>

Author: 3 people