[docs] Better describe difference role of @spawn in multithreading

https://docs.julialang.org/en/v1/base/multi-threading/#Base.Threads.@spawn says See the manual chapter on threading for important caveats. but in https://docs.julialang.org/en/v1/manual/multi-threading/ the @spawn macro is not used in any bigger example, not elaborated in more detail. Difference and similarity of @async and @spawn should be elaborated more, ideally with demonstration of both @async and @spawn being used with @sync. In https://docs.julialang.org/en/v1/base/parallel/#Base.@sync I see it can be used for @spawn too, but I think actually having example in manual would help much. Some related discussion here: https://julialang.slack.com/archives/C6SMTHQ3T/p1597182853179500