Update return values for GC.GetGeneration

[gewarren]

Contributes to dotnet/docs#35105.

[learn-build-service-prod]

Learn Build status updates of commit 21cf9f4:

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[learn-build-service-prod]

Learn Build status updates of commit e2aefab:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/GC.xml	✅Succeeded	View

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[learn-build-service-prod]

Learn Build status updates of commit cd07002:

✅ Validation status: passed

File	Status	Preview URL	Details
xml/System/GC.xml	✅Succeeded	View

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[BillWagner]

This LGTM @gewarren

Let's