CH01::Full node: clarify requirements; update "client" language to include "peer"

- Full nodes don't need to store transactions long-term or serve data to
  other software, so mention those as option

- Stop calling full nodes "clients".  Add a tipbox describing that full
  nodes are the peers on Bitcoin's P2P network.

ch01.asciidoc

@@ -231,15 +231,13 @@ hardware signing device.
 ===== Full-node vs. Lightweight
 Another way to categorize bitcoin wallets is by their degree of autonomy and how they interact with the Bitcoin network:
 
-Full-node client:: ((("full-node clients")))A full client, or "full
-node," is a client that stores the entire history of Bitcoin
-transactions (every transaction by every user, ever), manages users'
-wallets, and can initiate transactions directly on the Bitcoin network.
-A full node handles all aspects of the protocol and can independently
-validate the entire blockchain and any transaction. A full-node client
-consumes substantial computer resources (e.g., more than 125 GB of disk,
-2 GB of RAM) but offers complete autonomy and independent transaction
-verification.
+Full-node:: ((("full-node")))A full node is a program that validates the
+entire history of Bitcoin transactions (every transaction by every user,
+ever).  Optionally, full nodes can also store previously validated
+transactions and serve data to other Bitcoin programs, either on the
+same computer or over the internet.  A full node uses substantial
+computer resources--about the same as watching a streaming video for an
+hour--but offers complete autonomy to its users.
 
 Lightweight client:: ((("lightweight
 clients")))((("simplified-payment-verification (SPV)")))A lightweight
@@ -257,6 +255,19 @@ connecting to the Bitcoin network directly. The wallet may be stored by
 the user or by third-party servers, but all transactions go through a
 third party.
 
+[TIP]
+====
+Bitconin is a Peer-to-Peer (P2P) network.  Full nodes are the *peers:*
+each peer individually validates every confirmed transaction and can
+provide data to its user with complete authority.  Lightweight wallets
+and other software are *clients:* each client depends on one or more peers
+to provide it with valid data.  Bitcoin clients can perform secondary
+validation on some of the data they receive and make connections to
+multiple peers to reduce their depedence on the integrity of a single
+peer, but the security of a client ultimately relies on the integrity of
+its peers.
+====
+
 ===== Custodial vs. Non-Custodial
 
  _Your keys, your coins. Not your keys, not your coins_.