chore: add docs for why descendant checks are safe

crates/engine/tree/src/tree/mod.rs

@@ -2267,12 +2267,13 @@ where
 
         // TODO: switch to calculate state root using `StateRootTask`.
 
-        // We attempt to compute state root in parallel if we are currently not persisting anything
-        // to database. This is safe, because the database state cannot change until we
-        // finish parallel computation. It is important that nothing is being persisted as
-        // we are computing in parallel, because we initialize a different database transaction
-        // per thread and it might end up with a different view of the database.
-        // let persistence_in_progress = self.persistence_state.in_progress();
+        // We only run the parallel state root if we are currently persisting blocks that are all
+        // ancestors of the one we are executing. If we're committing ancestor blocks, then: any
+        // trie updates being committed are a subset of the in-memory trie updates collected before
+        // fetching reverts. So any diff in reverts (pre vs post commit) is already covered by the
+        // in-memory trie updates we collect in `compute_state_root_parallel`.
+        //
+        // See https://github.com/paradigmxyz/reth/issues/12688 for more details
         let is_descendant_block = self.persistence_state.current_action().map_or(true, |action| {
             match action {
                 CurrentPersistenceAction::SavingBlocks { blocks } => {