OpenBSD has a single PCI IDE driver for multiple vendors/chipsets, this includes SATA controllers in PCI IDE mode (..instead of AHCI).
The driver is full of quirks for initilizating all sorts devices, setting up channels, etc.. and there are buggy/undocumented chipsets out there that don't work properly as-is.
If you want to get this resolved, send a bug report using
sendbug(1) or send an email to bugs@ or even tech@.
This is not an issue with documentation, it is an issue with your chipset.