doc: document `pay` localofferid field.

Fixes: #4665
Reported-by: @shesek
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

contrib/pyln-client/pyln/client/lightning.py

@@ -997,7 +997,7 @@ class LightningRpc(UnixDomainSocketRpc):
 
     def pay(self, bolt11, msatoshi=None, label=None, riskfactor=None,
             maxfeepercent=None, retry_for=None,
-            maxdelay=None, exemptfee=None, exclude=None):
+            maxdelay=None, exemptfee=None, localofferid=None, exclude=None):
         """
         Send payment specified by {bolt11} with {msatoshi}
         (ignored if {bolt11} has an amount), optional {label}
@@ -1012,6 +1012,7 @@ class LightningRpc(UnixDomainSocketRpc):
             "retry_for": retry_for,
             "maxdelay": maxdelay,
             "exemptfee": exemptfee,
+            "localofferid": localofferid,
             "exclude": exclude,
         }
         return self.call("pay", payload)

doc/lightning-pay.7.md

@@ -6,7 +6,7 @@ SYNOPSIS
 
 **pay** *bolt11* [*msatoshi*] [*label*] [*riskfactor*]
 [*maxfeepercent*] [*retry\_for*] [*maxdelay*] [*exemptfee*]
-[*exclude*]
+[*localofferid*] [*exclude*]
 
 DESCRIPTION
 -----------
@@ -32,6 +32,11 @@ leveraged by forwarding nodes. Setting `exemptfee` allows the
 `maxfeepercent` check to be skipped on fees that are smaller than
 `exemptfee` (default: 5000 millisatoshi).
 
+`localofferid` is used by offers to link a payment attempt to a local
+`send_invoice` offer created by lightningd-offerout(7).  This ensures
+that we only make a single payment for an offer, and that the offer is
+marked `used` once paid.
+
 The response will occur when the payment fails or succeeds. Once a
 payment has succeeded, calls to **pay** with the same *bolt11* will
 succeed immediately.