gh-99875: Document rounding mode for old-style formatting (GH-126382)

(cherry picked from commit ce1abaf)

Co-authored-by: Sergey B Kirpichev <skirpichev@gmail.com>

Author: skirpichev

Doc/library/stdtypes.rst

@@ -3012,6 +3012,10 @@ The conversion types are:
 |            | character in the result.                            |       |
 +------------+-----------------------------------------------------+-------+
 
+For floating-point formats, the result should be correctly rounded to a given
+precision ``p`` of digits after the decimal point.  The rounding mode matches
+that of the :func:`round` builtin.
+
 Notes:
 
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