[Mingw-w64-public] Unwind info version 2: any documentation ?

Hello,

at least in ntdll.dll from Windows x64 2013, some UNWIND_INFO
records are using version 2.
But unfortunately, I wasn't able to get any documentation about
it.

That's a little bit worrying, as gdb is reading these tables
in order to compute the backtrace.  When it finds version 2
records, it cannot compute the previous frame.

Any pointers are welcome!

Regards,
Tristan.