chore(*): add mathlib4 synchronization comments (#18977)
Regenerated from the [port status wiki page](https://github.com/leanprover-community/mathlib/wiki/mathlib4-port-status).
Relates to the following files:
* `category_theory.functor.flat`
* `category_theory.preadditive.injective`
* `category_theory.preadditive.injective_resolution`
* `linear_algebra.projective_space.subspace`
* `measure_theory.measure.ae_measurable`
* `measure_theory.measure.measure_space`
* `measure_theory.measure.sub`
* `probability.probability_mass_function.basic`
* `topology.partition_of_unity`
