add list of supported scenarios

From grub2 point of view it is not really required; what is required, is sufficient space on disk with boot block (stage1) to install core.img (stage1.5) which is capable of finding where /boot is located.

From YaST point of view it is simply incorrect - there is no way even to express it in bootloader configuration dialog. if I say "Boot on MBR" it means - whatever disk comes first, not disk on which /boot is located. if I say "Boot on /boot" it means PBR, not MBR.