doc: document the payment_secret argument to sendpay.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

doc/lightning-sendpay.7

@@ -4,7 +4,7 @@ lightning-sendpay - Low-level command for sending a payment via a route
 .SH SYNOPSIS
 
 \fBsendpay\fR \fIroute\fR \fIpayment_hash\fR [\fIlabel\fR] [\fImsatoshi\fR]
-[\fIbolt11\fR] [\fIpartid\fR]
+[\fIbolt11\fR] [\fIpayment_secret\fR] [\fIpartid\fR]
 
 .SH DESCRIPTION
 
@@ -36,6 +36,12 @@ ending in \fImsat\fR or \fIsat\fR, or a number with three decimal places ending
 in \fIsat\fR, or a number with 1 to 11 decimal places ending in \fIbtc\fR\.
 
 
+The \fIpayment_secret\fR is the value that the final recipient requires to
+accept the payment, as defined by the \fBpayment_data\fR field in BOLT 4
+and the \fBs\fR field in the BOLT 11 invoice format\.  It is required if
+\fIpartid\fR is non-zero\.
+
+
 The \fIpartid\fR value, if provided and non-zero, allows for multiple parallel
 partial payments with the same \fIpayment_hash\fR\.  The \fImsatoshi\fR amount
 (which must be provided) for each \fBsendpay\fR with matching

doc/lightning-sendpay.7.md

@@ -5,7 +5,7 @@ SYNOPSIS
 --------
 
 **sendpay** *route* *payment\_hash* \[*label*\] \[*msatoshi*\]
-\[*bolt11*\] \[*partid*\]
+\[*bolt11*\] \[*payment_secret*\] \[*partid*\]
 
 DESCRIPTION
 -----------
@@ -33,6 +33,11 @@ amount to the destination. By default it is in millisatoshi precision; it can be
 ending in *msat* or *sat*, or a number with three decimal places ending
 in *sat*, or a number with 1 to 11 decimal places ending in *btc*.
 
+The *payment_secret* is the value that the final recipient requires to
+accept the payment, as defined by the `payment_data` field in BOLT 4
+and the `s` field in the BOLT 11 invoice format.  It is required if
+*partid* is non-zero.
+
 The *partid* value, if provided and non-zero, allows for multiple parallel
 partial payments with the same *payment_hash*.  The *msatoshi* amount
 (which must be provided) for each **sendpay** with matching