Update 810_prepare_multiple_iso.sh

Add a comment that describes the interdependency between
prep/NETFS/default/070_set_backup_archive.sh and
output/ISO/Linux-i386/810_prepare_multiple_iso.sh
that exists except for the mkrescue workflow.

usr/share/rear/output/ISO/Linux-i386/810_prepare_multiple_iso.sh

@@ -26,6 +26,11 @@ test "$ISO_MAX_SIZE" || return 0
 test "mkrescue" = "$WORKFLOW" && Error "The mkrescue workflow is forbidden when ISO_MAX_SIZE is set"
 
 local backup_path=$( url_path $BACKUP_URL )
+
+# The backuparchive variable value is set in prep/NETFS/default/070_set_backup_archive.sh
+# which is skipped in case of the mkrescue workflow but the mkrescue workflow is forbidden
+# when ISO_MAX_SIZE is set and this script is skipped when ISO_MAX_SIZE is not set
+# see https://github.com/rear/rear/pull/2063#issuecomment-469222487
 local isofs_path=$( dirname $backuparchive )
 
 # Because usr/sbin/rear sets 'shopt -s nullglob' the 'echo -n' command