bug#36594: No hint for missing argument

[Julien Lepiller]

When you type an option that requires an argument, but don't provide one, you 
get an error message that says there is a missing argument. There should 
brobably be a hint on what values are acceptable. For instance, there is no way 
to know from "guix package -v" or "guix package --help" what values are 
accepted. In other cases, there is a --list-something option.

Maybe -v is the only case where it would be useful to do something, because you 
could guess its existence and not use a verbosity level. Maybe fixing the help 
message to say --verbosity=[0-5] is enough?