libgetopts: tweak HasHarg/Occur docs

I found the current docs to be a little confusing. I believe this makes them straight to the point and less confusing.
In fact, it shouldn't. That's partly why I'm making this PR. With `Occur` set to `Multi`, it is allowed for the option not to show up at all. With this PR, I'm assuming that the code is correct but the docs outdated.