CH02::getting inputs: introduce UTXO term

Introduce the UTXO term as it is one of the most essential concepts to
understanding how Bitcoin works and is widely used in other Bitcoin
technical documentation.

ch02.asciidoc

@@ -347,13 +347,12 @@ wants to send to Bob. Most wallets keep track of all the available
 outputs belonging to addresses in the wallet. Therefore, Alice's wallet
 would contain a copy of the transaction output from Joe's transaction,
 which was created in exchange for cash (see <<getting_first_bitcoin>>).
-A bitcoin wallet application that runs as a full-node client actually
-contains a copy of every unspent output from every transaction in the
-blockchain. This allows a wallet to construct transaction inputs as well
-as quickly verify incoming transactions as having correct inputs.
-However, because a full-node client takes up a lot of disk space, most
+A bitcoin wallet application that runs on a full node actually
+contains a copy of every confirmed transaction's unspent outputs, called
+*Unspent Transaction Outputs* (UTXOs).
+However, because full nodes use more resources, most
 user wallets run "lightweight" clients that track only the user's own
-unspent outputs.
+UTXOs.
 
 If the wallet application does not maintain a copy of unspent
 transaction outputs, it can query the Bitcoin network to retrieve this