XML documentation comments and exception message of MemoryMarshal.Cast are inaccurate

[tfenise]

https://github.com/dotnet/runtime/blob/3bacc83b4f3d21df591cc25b667b5198d8cebcff/src/libraries/System.Private.CoreLib/src/System/Runtime/InteropServices/MemoryMarshal.cs#L102-L121
(also the ReadOnlySpan<> overload)

1. It doesn't just work on primitive types.
2. (Unmanaged) pointers are OK.
3. The error message with ThrowHelper.ThrowInvalidTypeWithPointersNotSupported is also inaccurate: "...Only value types without pointers or references are supported."

[dotnet-policy-service]

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[colejohnson66]

FYI: The XML documentation comments in this repo are not the ones used to generate the actual documentation. The actual documentation is in @dotnet/dotnet-api-docs.