Don't say that callbacks execute only at the end of the parent

[UmanShahzad]

That's misleading because they can be executed anywhere in the parent, even multiple times (indeed functions accepting "callbacks" that operate on array items will call the function on each iteration).

Simplify the wording here to explain only that the parent function can call it, without all the extra stuff which convolutes the definition.

[jsf-clabot]

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