improve help text

dev-utils/fai-kvm

@@ -54,7 +54,8 @@ usage() {
  -s <size>   size of the local disk (GB, defaults to 5GB)
  -d <num>    number of local disks (default is one)
  -m <mem>    RAM size (MB, defaults to 2000MB)
- -u <num>    number of the user (so multiple users can call this script)
+ -u <num>    user id. This will become the tap device number and a part of the MAC address
+             and part of the disk files names. Each VM must have a unique number.
  -V          do not use vga std with Bochs extensions, use the kvm default
  -U          Use kvm user networking instead of tap devices
 EOF
@@ -80,6 +81,7 @@ shift $(($OPTIND - 1))
 
 if [ -z "$1" ]; then
     echo Missing argument. pxe or cd
+    usage
     exit 1
 fi