Clarify XML comments for Utf8Parser.TryParse to reflect case insensitivity

[joegoldman2]

Summary

Same as dotnet/runtime#103861.

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

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Learn Build status updates of commit e796c47:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
xml/System.Buffers.Text/Utf8Parser.xml	:white_check_mark:Succeeded	View

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[gewarren]

Thanks @joegoldman2