Adds example to async_std::task::spawn

[tronta]

As I struggled using the function spawn properly (I didn't get that it can be executed without await :-( ), I'd like to add an other example.

[tronta]

Yes, the toplevel description. Would have helped me. In my opinion, more examples are better than less :grinning: What do you think, should be modified? Or do you suggest, this is enough...? Probably one needs to weigh up the maintenance effort.

[yoshuawuyts]

@tronta hah, yay glad that would've helped -- I would prefer we adapt the docs from std::thread for this instead -- putting this behavior into words is probably the right approach here.