In what year did they do that?

The same year the PC was released. The BIOS was listed in the tech reference manual appendix A.

http://classiccomputers.info/down/IBM/IBM_PC_5150/IBM_5150_T...

I recall it being part of the documentation that came with the original PCs.

> IBM published the entire listings of the BIOS for its original PC, PC XT, PC AT, and other contemporary PC models, in an appendix of the IBM PC Technical Reference Manual for each machine type.

https://en.wikipedia.org/wiki/BIOS