async is no longer the default

[PallHaraldsson]

Was changed in #208

[PallHaraldsson]

I see async is up to 13 times faster, i.e. down to 0.017 sec for me. I don't really care too much about the speed 0.2 sec. with sync is on by default after first use, just fixing the doc.

More importantly for first use what's possible (default options 24 sec combined, or 11x slower, or with async "only" 15 sec. for 7x slower):

$ ~/julia-1.7.0-rc3/bin/julia -O0 --compile=min

julia> @time using Blink
  0.916359 seconds (1.46 M allocations: 97.061 MiB, 2.58% gc time)

julia> @time (w = Window(async=true); body!(w, "Hello World"))
  1.113005 seconds (1.92 M allocations: 105.378 MiB, 6.26% gc time)