[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 #3 from Marco Rubini <marco.rubini08 at gmail dot com> ---
libstdc++ already exports __do_vformat_to as an external symbol, so it would be
a breakage to drop it in a future version. I don't know if it was intended or
not, but given that it is already exported, I think it would just be better to
have an explicit external declaration for it in <format>.