bsdinstall: Add note to zfsboot pointing to release/tools/vmimage.subr

The VM image creation scripts have a copy of this list of datasets which
should be kept in sync, which can otherwise be easily missed, as was the
case in the now-reverted 3e8619e535 ("bsdinstall: Create separate
datasets for directories in /usr/ports").

usr.sbin/bsdinstall/scripts/zfsboot

@@ -141,6 +141,7 @@ f_include $BSDCFG_SHARE/variable.subr
 #
 # NOTE: Requires /tmp, /var/tmp, /$ZFSBOOT_BEROOT_NAME/$ZFSBOOT_BOOTFS_NAME
 # NOTE: Anything after pound/hash character [#] is ignored as a comment.
+# NOTE: Keep in sync with release/tools/vmimage.subr
 #
 f_isset ZFSBOOT_DATASETS || ZFSBOOT_DATASETS="
 	# DATASET	OPTIONS (comma or space separated; or both)