consensus: mention Segregated Witness scheme in tx byte size comment

ouroboros-consensus/src/ouroboros-consensus/Ouroboros/Consensus/Ledger/SupportsMempool.hs

@@ -191,7 +191,9 @@ class ( Measure     (TxMeasure blk)
   -- in-block serialisation. Another example is the transaction of the
   -- hard-fork combinator which will include an envelope indicating its era
   -- when sent across the network. However, when embedded in the respective
-  -- era's block, there is no need for such envelope.
+  -- era's block, there is no need for such envelope. An example from upstream
+  -- is that the Cardano ledger's "Segregated Witness" encoding scheme
+  -- contributes to the encoding overhead.
   --
   -- INVARIANT Assuming no hash collisions, the size should be the same in any
   -- state in which the transaction is valid.