I only check the amd64 handbook part, but it should be fix in every installation guide that do that. Please note http://wiki.gentoo.org/wiki/Complete_Handbook/Finishing_off#Exiting_the_chrooted_environment doesn't have that problem. It's unsafe to lazy umount a live filesystem, so suggesting to lazy umount /mnt/gentoo/boot is dangerous for the user boot partition. As soon as /mnt/gentoo/boot is lazy umount, nothing prevent user from umounting /mnt/gentoo and rebooting, while /mnt/gentoo/boot might still be in use and need more time to finish works, endup in a damage filesystem. Handbook must be fix to : 1/ prevent damaging the user boot filesystem 2/ avoid teach user bad manner to use lazy umount instead of classic umount on any live filesystem. Hanbook is a teacher for many new users, if handbook teach them to lazy umount their boot, they might always lazy umount it later without knowning what they are doing. Reproducible: Always Seen because of http://forums.gentoo.org/viewtopic-t-997060.html thread that expose it.
Lazy umount is no longer used for the persistent directories