update docs with payment_preimage.

doc/lightning-invoice.7.md

@@ -67,8 +67,9 @@ RETURN VALUE
 ------------
 
 On success, a hash is returned as *payment\_hash* to be given to the
-payer, and the *expiry\_time* as a UNIX timestamp. It also returns a
-BOLT11 invoice as *bolt11* to be given to the payer.
+payer, the *payment\_preimage* used and the *expiry\_time* as a UNIX
+timestamp. It also returns a BOLT11 invoice as *bolt11* to be given
+to the payer.
 
 On failure, an error is returned and no invoice is created. If the
 lightning process fails before responding, the caller should use

doc/lightning-listinvoices.7.md

@@ -17,11 +17,11 @@ RETURN VALUE
 
 On success, an array *invoices* of objects is returned. Each object
 contains *label*, *payment\_hash*, *status* (one of *unpaid*, *paid* or
-*expired*), and *expiry\_time* (a UNIX timestamp). If the *msatoshi*
-argument to lightning-invoice(7) was not "any", there will be an
-*msatoshi* field as a number, and *amount\_msat* as the same number
-ending in *msat*. If the invoice *status* is *paid*, there will be a
-*pay\_index* field and an *msatoshi\_received* field (which may be
+*expired*), *payment\_preimage*, and *expiry\_time* (a UNIX timestamp).
+If the *msatoshi* argument to lightning-invoice(7) was not "any", there
+will be an *msatoshi* field as a number, and *amount\_msat* as the same
+number ending in *msat*. If the invoice *status* is *paid*, there will
+be a *pay\_index* field and an *msatoshi\_received* field (which may be
 slightly greater than *msatoshi* as some overpaying is permitted to
 allow clients to obscure payment paths); there will also be an
 *amount\_received\_msat* field with the same number as