gh-99875: document rounding mode for old-style formatting

[skirpichev]

• Issue: Document (undefined) rounding behaviour of printf-style formatting #99875

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📚 Documentation preview 📚: https://cpython-previews--126382.org.readthedocs.build/

[rruuaanng]

This description looks very good : --- )

[skirpichev]

CC @picnixz This is follow-up off #121481

[skirpichev]

CC @vstinner

[vstinner]

LGTM.

Do we have tests for that? test_format.test_precision() is quite basic, it doesn't check rounding.

        f = 1.2
        self.assertEqual(format(f, ".0f"), "1")
        self.assertEqual(format(f, ".3f"), "1.200")
        with self.assertRaises(ValueError) as cm:
            format(f, ".%sf" % (sys.maxsize + 1))

[skirpichev]

Do we have tests for that? test_format.test_precision() is quite basic, it doesn't check rounding.

We have tests in the test_float.py. E.g. test_format_testfile(). For instance:

%f 1.23456789 -> 1.234568

will fail with rounding up.

%.1f 1234.5678 -> 1234.6

will fail with rounding down. And so on.