chore(*): add mathlib4 synchronization comments (#19005)
Regenerated from the [port status wiki page](https://github.com/leanprover-community/mathlib/wiki/mathlib4-port-status).
Relates to the following files:
* `algebra.homology.short_exact.abelian`
* `algebra.modeq`
* `algebra.ring.boolean_ring`
* `analysis.locally_convex.continuous_of_bounded`
* `analysis.normed_space.units`
* `category_theory.abelian.diagram_lemmas.four`
* `category_theory.abelian.generator`
* `category_theory.idempotents.biproducts`
* `data.is_R_or_C.basic`
* `model_theory.definability`
* `model_theory.order`
* `model_theory.semantics`
* `model_theory.substructures`
* `topology.category.Top.limits.pullbacks`
