Following last week meeting, I updated D3258R0

Mark had some concerns here (I am starting a new thread to avoid having multiple interleaved discussions) https://lists.isocpp.org/sg16/2024/05/4302.php

After talking with Jonathan, we decided the best solution to address those concerns is to not modify basic_format_arg.

I  also fixed some latex formatting issues, and added some information in the "constexpr" section at Victor's request.

Thanks,

Corentin