Improve evaluateTransactionBalance documentation

lib/core/src/Cardano/Wallet/Transaction.hs

@@ -196,8 +196,12 @@ data TransactionLayer k tx = TransactionLayer
         -> UTxO
         -> [(TxIn, TxOut, Maybe (Hash "Datum"))] -- Extra UTxO
         -> Maybe Node.Value
-        -- ^ Evaluate the balance of a transaction using the ledger. A valid
-        -- transaction must be balanced.
+        -- ^ Evaluate the balance of a transaction using the ledger. The balance
+        -- can thought of as @Σ inputs - Σ outputs - fee@, but in reality
+        -- there are more features that can affect it, like minting, deposits,
+        -- and withdrawals. By relying on the calculation of the ledger, we
+        -- don't need to worry about getting all these details right. For a
+        -- transaction to be valid, it must have a balance of zero.
         --
         -- Note that the fee-field of the transaction affects the balance, and
         -- is not automatically the minimum fee.