`slcli globalip assign` should only take in an IP as an argument

`slcli globalip assign 12345678 9.111.123.456 ` this is the expected syntax since you can not route global ips to subnets/devices/etc.

Should still be able to support the `--target` and `--target-id` options of course. 
Update the help text to explain this a bit as well.

This command changed in #1614 