p2p: Improve nonce documentation

Better describe what the nonce is used for.

Note this file has not had its docs manicured so the line length is 80
still, just use the same line length instead of the conventional 100.

Fix: #575

bitcoin/src/p2p/message_network.rs

@@ -32,6 +32,13 @@ pub struct VersionMessage {
     /// The network address of the peer sending the message
     pub sender: Address,
     /// A random nonce used to detect loops in the network
+    ///
+    /// The nonce can be used to detect situations when a node accidentally
+    /// connects to itself. Set it to a random value and, in case of incoming
+    /// connections, compare the value - same values mean self-connection.
+    ///
+    /// If your application uses P2P to only fetch the data and doesn't listen
+    /// you may just set it to 0.
     pub nonce: u64,
     /// A string describing the peer's software
     pub user_agent: String,