[clang] [NFC] Add API documentation and annotations (PR #78635)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

It might be worth adding a .dox file something like this:
```
/*!
	\mainpage HLSL Overview
	
	\tableofcontents
	
	\section hlsl-introduction Introduction to HLSL
	- \ref hlsl_basic_types.h  defines hlsl types
	-\ref - hlsl_intrinsics.h  defines hlsl spec apis.

    \n
	\section hlsl-list HLSL Types
     - \ref float2-  A vector of 2 floats
     - \ref float3-  A vector of 3 floats
      - \ref float4-  A vector of 4 floats
 ```

https://github.com/llvm/llvm-project/pull/78635