Clarify what qualifies as valid transaction

cardano-api/internal/Cardano/Api/Experimental/Tx.hs

@@ -45,11 +45,15 @@ module Cardano.Api.Experimental.Tx
     -- ** Balancing a transaction
 
     -- |
-    -- If we have a UTxO with exactly 12 ADA, and 2 ADA is enough for the fees, we could just construct
-    -- the transaction like in the previous section directly, and it could be a valid transaction. But still,
-    -- even if we have exactly 12 ADA, it is unlikely the fees required are exactly 2 ADA, so either we made
-    -- an invalid transaction, or we are wasting ADA on fees. Also, we may not always want to send all the ADA
-    -- in the UTxO. So that is were transaction balancing comes in.
+    -- If we have a UTxO with exactly 12 ADA, we could just construct the transaction like in the
+    -- previous section directly, and it would be a valid transaction.
+    --
+    -- That is because the requirement is that the sum of inputs of the transactions is equal to
+    -- the sum of the outputs, and that the fees specified are enough, and 2 ADA is more than enough
+    -- for fees of such a simple transaction on mainnet at the time of writing. But still, even if we
+    -- have exactly 12 ADA, it is unlikely that the fees required are exactly 2 ADA, so we are likely
+    -- wasting ADA on fees. Also, we may not always want to send all the ADA in the UTxO. So that is
+    -- were transaction balancing comes in.
     --
     -- You can see how to balance a transaction in the documentation of the "Cardano.Api.Fees" module.
 

cardano-api/internal/Cardano/Api/Fees.hs

@@ -19,9 +19,12 @@ module Cardano.Api.Fees
     -- |
     -- The documentation of the "Cardano.Api.Tx.Body" module shows how to create a 'TxBodyContent' for a
     -- transaction that takes 12 ADA and sends 10 ADA to an address, and spends 2 ADA on fees. If we have a
-    -- UTxO with exactly 12 ADA, and 2 ADA is enough for the fees, this could be a valid transaction. But still,
-    -- even if we have exactly 12 ADA, it is unlikely that the fees required are exactly 2 ADA, so either we
-    -- made an invalid transaction, or we are wasting ADA on fees.
+    -- UTxO with exactly 12 ADA, this would be a valid transaction.
+    --
+    -- The requirement is that the sum of inputs of the transactions is equal to the sum of the outputs,
+    -- and that the fees specified are enough, and 2 ADA is more than enough for fees of such a simple
+    -- transaction on mainnet at the time of writing. But still, even if we have exactly 12 ADA, it is
+    -- unlikely that the fees required are exactly 2 ADA, so we are likely wasting ADA on fees.
     --
     -- On the other hand, we don't always want to spend all the ADA from a UTxO. Balancing a transaction allows
     -- us to send the amount we want to send, pay only the necessary fees (or little more), and it