add comment (#31252)

runtime/src/accounts_db.rs

@@ -4023,6 +4023,8 @@ impl AccountsDb {
                 .fetch_add(1, Ordering::Relaxed);
             for pubkey in shrink_collect.unrefed_pubkeys {
                 if let Some(locked_entry) = self.accounts_index.get_account_read_entry(pubkey) {
+                    // pubkeys in `unrefed_pubkeys` were unref'd in `shrink_collect` above under the assumption that we would shrink everything.
+                    // Since shrink is not occurring, we need to addref the pubkeys to get the system back to the prior state since the account still exists at this slot.
                     locked_entry.addref();
                 }
             }