Add missing xml docs for System.Runtime.Intrinsics (#11930)

* Add missing xml docs for System.Runtime.Intrinsics

* Apply suggestions from code review

Co-authored-by: Copilot <[email protected]>

* Address review feedback

* Resolve more PR feedback

* Remove docs for the new extension properties, tooling support is coming soon

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Co-authored-by: Copilot <[email protected]>

Author: tannergooding