epm: print comment about parallel when run with --parallel only

bin/epm

@@ -752,7 +752,7 @@ esac
 
 case $epm_cmd in
     upgrade|Upgrade|install|reinstall|Install|release_upgrade|release_downgrade)
-        if [ -z "$eget_backend" ] ; then
+        if [ -n "$parallel" ] && [ -z "$eget_backend" ] ; then
             is_command aria2 && eget_backend=aria2 && echo "Use installed aria2 to parallel package downloading."
             [ -z "$eget_backend" ] && is_command axel && eget_backend=axel && echo "Use installed axel to parallel package downloading."
             [ -n "$eget_backend" ] || info "It is better to install aria2 for parallel downloading."