doc: add explanation why keep var with for loop in async_hooks
Refs: nceu19-async_hooks
This comment will help contributors to understand why keeping var in some for loop instead changing at to let, as discussed with @mcollina
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines