[docs] [issue33082] multiprocessing docs bury very important 'callback=' args

[Antoine Pitrou]

Antoine Pitrou <pitrou at free.fr> added the comment:

I don't really agree that "callbacks are really important in multiprocessing" (I think I've hardly ever used them).  I agree with the other doc changes in the PR.

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<https://bugs.python.org/issue33082>
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