[clang] [clang] Update diagnostics for type aware allocators (PR #148576)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> Please update `clang/docs/LanguageExtensions.rst`.

Good point, also as it got dropped from C++26 I'm restoring and updating the in tree docs as well.

https://github.com/llvm/llvm-project/pull/148576