Potential very minor improvement in styling: accepting || {Doc} closures

[eugenesvk]

I was trying to replicate the with_usage example of having formatted text in other fields like short/long, but couldn't, the copy&pasted || {} style didn't work.
I had to recreate Doc in a separate variable and pass it to the options before the obvious solution dawned on me that I could just use {} in-place as well

So just an idea: maybe still the signature of long/short could also accept closures generating Doc?

.header("Sometimes they even work.")
        .footer("Beware `-d`, dragons be here")
        .with_usage(|doc| {
            let mut u = Doc::default();