Update fmt/std.h section

[tupaschoal]

This is just trying to keep the std.h documentation up-to-date. I just opened https://github.com/tupaschoal/fmt/blob/7ba6420540383f378c48834beb46f9cec5504dd2/include/fmt/std.h and looked through all formatter specializations:

std::bitset https://github.com/tupaschoal/fmt/blob/master/include/fmt/std.h#L172
std::source_location https://github.com/tupaschoal/fmt/blob/master/include/fmt/std.h#L248
std::error_code https://github.com/tupaschoal/fmt/blob/master/include/fmt/std.h#L365
std::atomic https://github.com/tupaschoal/fmt/blob/master/include/fmt/std.h#L514
std::atomic_flag https://github.com/tupaschoal/fmt/blob/master/include/fmt/std.h#L527

PS: I added an optional commit to sort the section, because as it grows I think it gets easier to read

[vitaut]

Thank you!