[Bug libstdc++/126675] Document std::__format::__do_vformat_to or make the explicit instantiation extern

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126675

--- Comment #7 from Tomasz Kamiński <tkaminsk at gcc dot gnu.org> ---
> Existing binaries compiled with -std=c++20 rely on the symbol being exported by libstdc++ or they will fail to link. libstdc++ users are not aware of this dependency because the explicit template declaration was not written by them.

The exported template has additional unsigned template parameter, and we could
decided export an different symbol in C++23, that will be later used if TU is
compiled in C++23 mode. What we commited to is that C++20 TU will use symbols
with value 1u.