docs: note std::format vs fmt difference for 'a' format

[leno23]

Fixes #4657

Adds a bullet to the "differences from std::format" section in doc/api.md explaining that 'a'/'A' output in {fmt} includes the 0x/0X prefix (like printf %a), while std::format hexadecimal floating-point formatting via std::to_chars does not.

Example: https://godbolt.org/z/TaK4s5YTM

Made with Cursor

[leno23]

Friendly bump: this documents the 0x prefix difference for 'a' format vs std::format (see Godbolt in #4657). Happy to adjust wording if maintainers prefer a different section.

[leno23]

Docs-only clarification for #4657 — ready for review when you have time.

[leno23]

Docs clarification for 'a' format — ready for review.

[leno23]

Docs for 'a' format — ready for review when convenient.

[leno23]

Clarifies 'a' vs std::format — docs-only, CI green. Ready for review.

[leno23]

Docs-only — CI green, ready for review.