UI: clarify in the CLI help that `runtime` and `init-image` are exclusive

[delabassee]

The CLI init help doesn't mention that runtime and init-image args are exclusive.
-> fn [global options] init [command options] [function-subdirectory]

I'm not sure about the exact syntax with -- but something like this would be more clear
fn [global options] init --{runtime|init-image} [command options] [function-subdirectory]