Add missing docs to FileStream.CopyTo(Stream, int)

[jozkee]

Contributes to dotnet/runtime#105979 (comment).

[dotnet-policy-service]

Tagging subscribers to this area: @dotnet/area-system-io

[learn-build-service-prod]

Learn Build status updates of commit 3852d1e:

❌ Validation status: errors

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	❌Error		Details

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[learn-build-service-prod]

Learn Build status updates of commit 3852d1e:

✅ Validation status: passed

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

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