Add expiration write fee description.

core/cap-0046-07.md

@@ -378,7 +378,7 @@ Rent fee has to be payed if operation increases the lifetime of the ledger entri
 
 Rent fee is computed only at transaction application time and it depends on the state of the ledger entries before and after the transaction has been applied.
 
-Fee: `Rent_fee = sum(rent_fee_per_entry_change(entry_before, entry_after))` for all the ledger entry changes.
+Fee: `Rent_fee = sum(rent_fee_per_entry_change(entry_before, entry_after)) + expiration_write_fee` for all the ledger entry changes.
 
 Entry rent fee consists of two components: fee for renting new ledgers with the new entry size and fee for renting the old ledgers with increased size. If `entry_before` does not exist, we treat its size as `0`` and expiration ledger as `0` for the sake of this formula.
 
@@ -410,6 +410,16 @@ rent_fee_for_size_and_ledgers(is_persistent, S, L) = round_up(
 
 Settings values come from `StateExpirationSettings`.
 
+Additionally, we charge for the `ExpirationEntry` writes of entries that had `expirationLedger` changed using the same rate as for any other entry write:
+
+```
+expiration_write_fee =  
+  num_expiration_updates * feeWriteLedgerEntry +
+  round_up(write_fee_per_1kb(BucketListSize) * EXPIRATION_ENTRY_SIZE / 1024)
+```
+
+where `num_expiration_updates` is the number of ledger entries that had expiration ledger updated and `EXPIRATION_ENTRY_SIZE` is size of `ExpirationEntry` with its key and is set to `68` bytes.
+
 Validity constraints: _None_
 
 Apply-time enforcement: _None_