[clang] [clang][docs] Fix DR status for P2280R4 (PR #127166)

[Vlad Serebrennikov via cfe-commits]

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@@ -510,6 +510,12 @@ <h2 id="cxx23">C++23 implementation status</h2>
       <td class="full" align="center">Yes</td>
     </tr>
 </table>
+
+<p>
+<span id="p2280">(12): In Clang 20, this change is not yet retroactively
----------------
Endilll wrote:

```suggestion
<span id="p2280">(12): This feature is not yet available in C++20 and earlier modes.
```

https://github.com/llvm/llvm-project/pull/127166