Was looking myself and it seems someone has managed to find and upload a document here:

https://kib.kiev.ua/x86docs/AMD/MISC/19725c_opic_spec_1.2_oct95.pdf

Sadly I could find no more official source, but hopefully this leads to a few more people possessing a copy for the next time it disappears. In the interest of better preserving it, I may attempt to somehow attach or link it here (though I have no affiliation with the site, it seems we were all looking for the same thing):
https://wiki.osdev.org/Open_Programmable_Interrupt_Controller