Follow-up of #19 and more documentation

[Draphar]

After the change to as_millis for Windows, I changed it here as well. Because the method is only supported since Rust 1.33, the 1.21 build on Travis is failing. Is that a problem?

I also added a little bit more information about the underlying mechanisms and their limitations as you suggested.

[alexcrichton]

Thanks for the PR! I'm not sure that this is quite right though? The intention is that it's valid to pass in any Duration and we'll simply loop until the duration lapses, and if that involves multiple system calls that should be fine (as they're on the order of years apart anyway)

[Draphar]

Sorry, could you eloborate on the problem? My intention was just to use replace manual conversion to milliseconds with the dedicated method in std. Everything else was supposed to be kept intact.

[alexcrichton]

Oh sure, so this is accidentally introducing a behavior change where if a long duration is specified it's truncated and/or panics. The intention is that all durations are valid (there are no max durations) and we just handle it in each implementation.

[Draphar]

That certainly wasn't the intention. Do you want to keep the documentation?

[alexcrichton]

I think the documentation is unfortunately slightly incorrect as well because there's no maximum timeout when calling these methods