The section should also mention that you cannot use guided partitioning (except free space) for the disk that you boot the installer from and that the CD-image is on.
Maybe also partman-auto should check for this instead of leaving it up to libparted to throw an error: ! Partman: /dev/scsi/host0/bus1/target0/lun0/part1 is mounted; will not ! make a filesystem here! Not sure if it is worth coding this as the method is not used that often and it is kind of obvious. Probably documenting it is sufficient.
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