pythongh-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().

Doc/library/string.rst

@@ -588,6 +588,13 @@ The available presentation types for :class:`float` and
    |         | as altered by the other format modifiers.                |
    +---------+----------------------------------------------------------+
 
+The result should be correctly rounded to a given precision ``p`` of digits
+after the decimal point.  For :class:`float`, used rounding mode match the one
+for :func:`round` builtin, i.e. it should round results midway between
+representable values toward the nearest value with an even (0) least
+significant bit.  For :class:`~decimal.Decimal`, rounding mode of the current
+:ref:`context <decimal-context>` is used.
+
 
 .. _formatexamples: