fixup: explain use of wal_coin_balances

crates/sui-indexer-alt/migrations/2024-10-30-232206_wal_coin_balances/up.sql

@@ -15,6 +15,17 @@
 -- - There is an additional index on ID and version for querying the latest
 --   version of every object.
 --
+-- This table is used in conjunction with `sum_coin_balances` to support
+-- consistent live object set queries: `sum_coin_balances` holds the state of
+-- the live object set at some checkpoint `C < T` where `T` is the tip of the
+-- chain, and `wal_coin_balances` stores all the updates and deletes between
+-- `C` and `T`.
+--
+-- To reconstruct the the live object set at some snapshot checkpoint `S`
+-- between `C` and `T`, a query can be constructed that starts with the set
+-- from `sum_coin_balances` and adds updates in `wal_coin_balances` from
+-- `cp_sequence_number <= S`.
+--
 -- See `up.sql` for the original `sum_coin_balances` table for documentation on
 -- columns.
 CREATE TABLE IF NOT EXISTS wal_coin_balances