Let me quote the description about CONFIG_BLK_DEV_IDE_PMAC_ATA100FIRST
This option will cause the ATA/100 controller found in UniNorth2
based machines (Windtunnel PowerMac, Aluminium PowerBooks, ...)
to be probed before the ATA/66 and ATA/33 controllers. Without
these, those machine used to have the hard disk on hdc and the
CD-ROM on hda. This option changes this to more natural hda for
hard disk and hdc for CD-ROM.
Device drivers -> IDE ->
[*] Builtin PowerMac IDE support
[*] Probe internal ATA/100 (Kauai) first
Sometimes users got problems in booting the first time because of that.
Sorry, but the installation instructions shouldn't start dealing with possible quircks people can come across. It's a generic installation line. For this kind of support, I believe the forums and IRC are a better medium, or perhaps a support-specific database.