add a comment explaining why /dev cleaning is docker-only

live-build/auto/build

@@ -266,8 +266,11 @@ EOF
 		# Save even more size by removing apt lists (that are currently removed
 		# downstream anyway)
 		rm -rf chroot/var/lib/apt/lists/*
-		# Having device notes in the docker image can cause problems:
-		# https://github.com/tianon/docker-brew-ubuntu-core/issues/62
+		# Having device notes in the docker image can cause problems
+		# (https://github.com/tianon/docker-brew-ubuntu-core/issues/62)
+		# so remove them.  We only do this for docker because the rootfs
+		# for system booting without initramfs needs a minimal /dev to
+		# get to the point of running udev.
 		rm -rf chroot/dev/*
 	fi