[clang] [HLSL] Document all spellable attributes (PR #213346)

[Justin Bogner via cfe-commits]

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@@ -8995,6 +8995,23 @@ for details.
   }];
 }
 
+def HLSLControlFlowHintDocs : Documentation {
+  let Category = DocCatStmt;
+  let Heading = "branch, flatten";
+  let Content = [{
+The ``branch`` and ``flatten`` attributes can be applied to *if statements* in
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bogner wrote:

Yes that's true. I'll update this.

https://github.com/llvm/llvm-project/pull/213346