Add missing xml docs for System.Runtime.Intrinsics

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This resolves dotnet/runtime#120508

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[Copilot]

Pull Request Overview

Adds and enriches missing XML documentation for System.Runtime.Intrinsics vector types (Vector64, Vector128, Vector256, Vector512) to address issue #120508.

• Replaces many placeholder "To be added." entries with meaningful summaries, type parameter, parameter, and returns descriptions.
• Adds references for constant properties and removes some inheritdoc tags where explicit docs are now provided.
• Introduces new remarks and exception tags, though a few contain grammatical and structural issues.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 13 comments.

File	Description
xml/System.Runtime.Intrinsics/Vector64.xml	Adds inheritdoc references for constant getters; enhances classification method docs.
xml/System.Runtime.Intrinsics/Vector128.xml	Substantial expansion of summaries, parameters, returns, and exceptions; removes some inheritdoc tags.
xml/System.Runtime.Intrinsics/Vector256.xml	Mirrors Vector64 constant inheritdoc additions and removes some inheritdoc tags.
xml/System.Runtime.Intrinsics/Vector512.xml	Similar updates to Vector256 with inheritdoc additions and cleanup.

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