whatsnew for gc.get_stats, plus doc tweaks.

Clarified the "At the moment" wording, and added the get_stats entry in the
module summary that Serhiy noted was missing at the end of issue 16351.

Given that pydoc lists all the function docstrings, I'm not sure that module
summary section is actually needed; but, it is probably better to address that
when the module is converted to use Argument Clinic.  In the meantime we
should keep the list complete.