ch-03: mention vbytes is the main accounting unit for fees

03_how_ln_works.asciidoc

@@ -716,7 +716,7 @@ Hence, capacity and connectivity are critical and scarce resources in the Lightn
 ==== Incentives for large value payment vs. small value payments
 
 The fee structure in Bitcoin is independent of the transaction value.
-A $1 million transaction has the same fee as a $1 transaction on Bitcoin, assuming a similar transaction size in bytes.
+A $1 million transaction has the same fee as a $1 transaction on Bitcoin, assuming a similar transaction size in bytes (more specifically "virtual" bytes after segwit).
 In Lightning the fee is a fixed base fee plus a percentage of the transaction value.
 Therefore, in Lightning the payment fee increases with payment value.
 These opposing fee structures create different incentives and lead to different usage in regards to transaction value.