chore(*): add mathlib4 synchronization comments (#18623)
Regenerated from the [port status wiki page](https://github.com/leanprover-community/mathlib/wiki/mathlib4-port-status).
Relates to the following files:
* `algebra.dual_number`
* `data.finsupp.well_founded`
* `data.nat.multiplicity`
* `data.polynomial.div`
* `data.zmod.algebra`
* `data.zmod.parity`
* `ring_theory.ideal.idempotent_fg`
* `ring_theory.mv_polynomial.symmetric`
