[BUG]: Description of is_fmt_numeric does not match actual behavior

[MaartenBaert]

Required prerequisites

• Make sure you've read the documentation. Your issue may be addressed there.
• Search the issue tracker and Discussions to verify that this hasn't already been reported. +1 or comment there if it has.
• Consider asking first in the Gitter chat room or in a Discussion.

What version (or hash if on master) of pybind11 are you using?

75e48c5

Problem description

The description of is_fmt_numeric states (in comments):

Note that the long double types only participate when long double is actually longer than double (it isn't under MSVC).

However this isn't what actually happens, because std::is_same<double, long double>::value is false even when sizeof(double) == sizeof(long double), as is indeed the case with MSVC on x86_64:
https://godbolt.org/z/xdd135a8T

Likewise, std::is_same<int, long>::value is false even when sizeof(int) == sizeof(long). This is presumably why platform_lookup uses sizeof rather than std::is_same to check type equivalence.

I'm not sure whether the correct solution here is to update the description to match actual behavior, or rather change the behavior to match the description.

Reproducible example code

int main() {
    std::cerr << sizeof(float) << ' ' << detail::is_fmt_numeric<float>::index << std::endl;
    std::cerr << sizeof(double) << ' ' << detail::is_fmt_numeric<double>::index << std::endl;
    std::cerr << sizeof(long double) << ' ' << detail::is_fmt_numeric<long double>::index << std::endl;
};

// expected output:
// 4 9
// 8 10
// 8 10

// actual output:
// 4 9
// 8 10
// 8 11

Is this a regression? Put the last known working version here if it is.

Not a regression