f reword docs on expiry val suggested

Author: TheBlueMatt

lightning/src/ln/channelmanager.rs

@@ -3458,6 +3458,10 @@ impl<Signer: Sign, M: Deref, T: Deref, K: Deref, F: Deref, L: Deref> ChannelMana
 	///
 	/// `invoice_expiry_delta_secs` describes the number of seconds that the invoice is valid for
 	/// in excess of the current time. This should roughly match the expiry time set in the invoice.
+	/// After this many seconds, we will remove the inbound payment, resulting in any attempts to
+	/// pay the invoice failing. The BOLT spec suggests 7,200 secs as a default validity time for
+	/// invoices when no timeout is set.
+	///
 	/// Note that we use block header time to time-out pending inbound payments (with some margin
 	/// to compensate for the inaccuracy of block header timestamps). Thus, in practice we will
 	/// accept a payment and generate a [`PaymentReceived`] event for some time after the expiry.