Fix Selected Permissions overview

[jlaundry]

When trying to add SharePoint Selected Permissions to an application, we struggled, as we found:

• It was unclear that these APIs are only available via the beta version
• That the displayName attribute is now required when assigning permissions

This short PR clarifies these points.

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[learn-build-service-prod]

Learn Build status updates of commit 717d774:

✅ Validation status: passed

File	Status	Preview URL	Details
concepts/permissions-selected-overview.md	✅Succeeded

For more details, please refer to the build report.

[Danipocket]

@jlaundry, are you a MSFTE? I need to validate some things. Thanks!

[jlaundry]

@Danipocket I'm not, but you can find more info and my contact details in support case 2511260030008384