docs(litertals): add comment on compiler differences

[vaeng]

          Yeah, I should have probably opened a separate PR for this. The `d` suffix is not part of the [C++ standard](https://en.cppreference.com/w/cpp/language/floating_literal) it seems:

suffix, if present, is one of f, l, F, L, f16, f32, f64, f128, bf16, F16, F32, F64, F128, BF16(since C++23).

ChatGPT claims it was a common thing, but I have never seen it before and apparently it is only supported by GCC. Clang and MSVC report an error.

Originally posted by @ahans in #856 (comment)