This PR fixes an out-of-date note attached to int conversions in stdtypes.rst.

The "may round or truncate as in C" language dates from almost 30 years ago, when conversion from a Python float to a Python int involved at C level a (long) cast applied to a C double value. Prior to standard C, presumably the precise behaviour of that cast couldn't be relied upon. But since C89 standardised this behaviour and we can reasonably rely on all current C compilers adhering to that or later standards, the note is out of date. (Moreover, the current code for converting a Python 3 int to a C double is very different.)

Related discuss.python.org thread: https://discuss.python.org/t/built-in-types-int-round-or-truncate-please-elaborate/24840