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

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines