gh-99880: document rounding mode for new-style formatting (GH-121481)

* gh-99880: document rounding mode for new-style formatting

The CPython uses _Py_dg_dtoa(), which does rounding to nearest with half
to even tie-breaking rule.

If that functions is unavailable, PyOS_double_to_string() fallbacks to
system snprintf().  Since CPython 3.12, build requirements include C11
compiler *and* support for IEEE 754 floating point numbers (Annex F).
This means that FE_TONEAREST macro is available and, per default,
printf-like functions should use same rounding mode as _Py_dg_dtoa().

* Update Doc/library/string.rst

Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>

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Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>