Modify some Capella comments (#4015)

* Modify comment to only include 4844

Capella only modifies per epoch processing by adding
`process_historical_summaries_update`, which does not change the realization of
justification or finality.

Whilst 4844 does not currently modify realization, the spec is not yet final
enough to say that it never will.

* Clarify address change verification comment

The verification of the address change doesn't really have anything to do with
the current epoch. I think this was just a copy-paste from a function like
`verify_exit`.

consensus/fork_choice/src/fork_choice.rs

@@ -857,8 +857,8 @@ where
                     (parent_justified, parent_finalized)
                 } else {
                     let justification_and_finalization_state = match block {
-                        // FIXME: verify this is correct for Capella/Eip4844 because
-                        //        epoch processing changes in Capella..
+                        // TODO(eip4844): Ensure that the final specification
+                        // does not substantially modify per epoch processing.
                         BeaconBlockRef::Eip4844(_)
                         | BeaconBlockRef::Capella(_)
                         | BeaconBlockRef::Merge(_)