Fix various inheritdoc so there is only "one indirection"

[tannergooding]

This resolves #11957 and resolves dotnet/runtime#121689

[dotnet-policy-service]

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

[Copilot]

Pull Request Overview

This PR refactors XML documentation to eliminate multiple levels of indirection in <inheritdoc> references. The changes ensure that all vector types (Vector64, Vector512, and System.Numerics vectors) now directly reference Vector128 as the canonical documentation source, rather than creating documentation inheritance chains.

Key changes:

• Vector128 now contains the full documentation (replacing "To be added" placeholders)
• Vector64 and Vector512 now use "To be added" + inheritdoc references pointing directly to Vector128
• System.Numerics vector types (Vector2, Vector3, Vector4, Vector, Vector`1) updated to reference Vector128 intrinsics instead of Vector4
• Corrected FusedMultiplyAdd references that incorrectly pointed to MultiplyAddEstimate
• Removed duplicate/incorrect inheritdoc tags

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
xml/System.Runtime.Intrinsics/Vector64.xml	Changed from full documentation to "To be added" + inheritdoc references to Vector128
xml/System.Runtime.Intrinsics/Vector512.xml	Updated inheritdoc references from Vector256 to Vector128
xml/System.Runtime.Intrinsics/Vector128.xml	Now contains full documentation (replacing "To be added" placeholders and removing Vector64 inheritdoc references)
xml/System.Numerics/Vector`1.xml	Fixed incorrect summary and returns descriptions for inequality operator
xml/System.Numerics/Vector4.xml	Updated inheritdoc references to point to Vector128, removed duplicate inheritdoc, corrected FusedMultiplyAdd reference
xml/System.Numerics/Vector3.xml	Updated inheritdoc references from Vector4 to Vector128 intrinsics
xml/System.Numerics/Vector2.xml	Updated inheritdoc references from Vector4 to Vector128 intrinsics
xml/System.Numerics/Vector.xml	Updated inheritdoc references from Vector4 to Vector128 intrinsics

[gewarren]

Left a few suggestions for Vector128 docs.

[tannergooding]

GitHub continues to not like applying suggestions to files that require clicking Load diff - Large diffs are not rendered by default., so I had to manually apply them locally instead

[tannergooding]

@gewarren, does anything need to happen to ensure these get published for .NET 10 or will that happen automatically at this point?

-- These are all cases that were "undocumented" due to the inheritdoc -> inheritdoc -> actual docs scenario not being supported

[gewarren]

@gewarren, does anything need to happen to ensure these get published for .NET 10 or will that happen automatically at this point?

-- These are all cases that were "undocumented" due to the inheritdoc -> inheritdoc -> actual docs scenario not being supported

@tannergooding Once #12057 merges in about 25 minutes, the docs will be live :D. Example you can check: https://learn.microsoft.com/en-us/dotnet/api/system.numerics.vector.extractmostsignificantbits?view=net-10.0#system-numerics-vector-extractmostsignificantbits(system-numerics-vector2)