!fixup add attr docs.

Author: fhahn

clang/include/clang/Basic/AttrDocs.td

@@ -9242,3 +9242,14 @@ Declares that a function potentially allocates heap memory, and prevents any pot
 of ``nonallocating`` by the compiler.
   }];
 }
+
+def ValueTypeDocs : Documentation {
+  let Category = DocCatDecl;
+  let Content = [{
+The ``value_type`` attribute can be used to mark user-defined types as 'value
+types'. When objects of value types are passed by-value to functions, the
+argument must be the only value referencing the passed object. This allows the
+optimizer to assume that no other pointers may alias the object when it is
+passed indirectly to a function..
+  }];
+}