[Bug c++/86094] [8/9 Regression] Call ABI changed for small objects with defaulted ctor

[jason at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86094

--- Comment #4 from Jason Merrill <jason at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #3)
> -fabi-version=12 is not documented, not mentioned in release notes, and not
> wired up in -Wabi.

-Wabi=11 warns about this.  With an incorrect message, since this is a bug. 
I'll also fix the documentation.