Resolve an ambiguity concerning fees.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

zip-0308.rst

@@ -188,7 +188,7 @@ section. It includes the time spent waiting for the first batch to be sent.
 How much to send in each transaction
 ------------------------------------
 
-If the remaining amount is less than 0.01 ZEC, end the migration.
+If the remaining amount to be migrated is less than 0.01 ZEC, end the migration.
 
 Otherwise, the amount to send in each transaction is chosen according to the
 following distribution:
@@ -202,6 +202,9 @@ Implementations MAY optimize this procedure by selecting the exponent and
 mantissa based on the amount remaining to avoid repetition, but the resulting
 distribution MUST be identical.
 
+The amount chosen *includes* the 0.0001 ZEC fee for this transaction, i.e.
+the value of the Sapling output will be 0.0001 ZEC less.
+
 Rationale
 '''''''''
 
@@ -217,6 +220,10 @@ of transactions, including "incorrect" solutions (that is, solutions that mix
 transactions from different users, contrary to the supposed adversary's
 inference).
 
+Making the chosen amount inclusive of the fee avoids leaving any unmigrated
+funds at the end, in the case where the original amount to migrate was a
+multiple of 0.01 ZEC.
+
 
 Other design decisions
 ----------------------
@@ -230,7 +237,8 @@ There will always be exactly one Sapling Output.
 
 The expiration height is chosen as for other transactions.
 
-The fee for each migration transaction MUST be 0.0001 ZEC.
+The fee for each migration transaction MUST be 0.0001 ZEC. This fee is taken
+from the funds to be migrated.
 
 Some wallets by default add a "developer fee" to each transaction, directed
 to the developer(s) of the wallet. This is typically implemented by adding