[HLSL][Doc] Add doc about expected differences (#82395)

This document covers expected differences between Clang and the HLSL
reference compiler implementations (FXC & DXC). The document is not
intended to be exhaustive, but it should be a best effort to cover known
cases.

This document should document both the behavioral difference and the
explanation of why Clang differs.

The initail document covers known overload resolution differences.

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Co-authored-by: S. Bharadwaj Yadavalli <Bharadwaj.Yadavalli@microsoft.com>