[Bug middle-end/113508] widen_ssumm3 documentation needs to mention which mode is m here

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=113508

--- Comment #2 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
There are few other changes I am going to make too.