[clang] Document the warn_unused attribute (PR #201881)

[Erich Keane via cfe-commits]

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@@ -388,6 +388,40 @@ pointer, but not any provenance of the allocation:
   }];
 }
 
+def WarnUnusedDocs : Documentation {
+  let Category = DocCatType;
+  let Content = [{
+The ``warn_unused`` attribute can be placed on the declaration of a structure or union type.
+When the ``-Wunused-variable`` diagnostic is enabled, local variables of types which have a non-trivial constructor or destructor are considered "used" by virtue of the constructor or destructor invocations involved.
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erichkeane wrote:

Do we not use column-limits in these files?  

https://github.com/llvm/llvm-project/pull/201881