[clang] [HLSL][Docs] Add documentation for HLSL functions (PR #75397)

[Damyan Pepper via cfe-commits]

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@@ -0,0 +1,300 @@
+===================
+HLSL Function Calls
+===================
+
+.. contents::
+   :local:
+
+Introduction
+============
+
+This document descries the design and implementation of HLSL's function call
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damyanp wrote:

typo: `descries` -> `describes`

https://github.com/llvm/llvm-project/pull/75397