async: new solution for finding current worker
async: new solution for finding current worker

Updated the documentation with a more simple solution for retrieving the
current worker of a task and directing a subtask to that same worker.

Also added a code example.

Signed-off-by: Stefano Zacchiroli <d7cdf09fc0f0426e98c9978ee42da5d61fa54986@upsilon.cc>

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