chore(*): add mathlib4 synchronization comments (#18621)
Regenerated from the [port status wiki page](https://github.com/leanprover-community/mathlib/wiki/mathlib4-port-status).
Relates to the following files:
* `algebra.group_ring_action.invariant`
* `algebra.quadratic_discriminant`
* `data.dfinsupp.well_founded`
* `data.mv_polynomial.comm_ring`
* `data.mv_polynomial.invertible`
* `data.mv_polynomial.supported`
* `data.mv_polynomial.variables`
* `data.zmod.basic`
* `ring_theory.finiteness`
* `ring_theory.multiplicity`
* `ring_theory.polynomial.chebyshev`
* `ring_theory.valuation.integers`
* `topology.category.Born`
