if no description is provided, allow first argument to example to take up entire line

Hi,

I'm not sure what the problem is, but over on [etcher](https://etcher.io) today I found a problem where yargs seemed to be applying the line-wrapping over-enthusiastically?
https://github.com/resin-io/etcher/issues/810

@jviotti managed to work around it, but I figured it would only be 'polite' to also report the problem upstream ;-)