Update Test-ADServiceAccount.md

[HerbertMauerer]

clarification on GMSA

PR Checklist

• [ ] Descriptive Title: Adding clarifications on when this cmdlets scan be used..
• [ ] Summary: this cmdlet is mainly for MSAs, not GMSAs.
• [ ] Contributor's Guide: I have read the contributors guide.
• [ ] Style: This PR adheres to the style guide.

[learn-build-service-prod[bot]]

Learn Build status updates of commit 191de7f:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
docset/winserver2022-ps/activedirectory/Test-ADServiceAccount.md	:white_check_mark:Succeeded	View (WindowsServer2022-ps)

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