update docs with payment_preimage.

Changelog-Added: JSON-RPC: `listinvoices` now displays the payment preimage if the invoice was paid.

doc/lightning-listinvoices.7.md

@@ -15,17 +15,16 @@ if it exists, or the status of all invoices if given no argument.
 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
-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
-*msatoshi\_received* but ending in *msat*.
+On success, an array *invoices* of objects is returned. Each object contains
+*label*, *payment\_hash*, *status* (one of *unpaid*, *paid* or *expired*),
+*payment\_preimage* (for paid invoices), 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 *msatoshi\_received* but ending in *msat*.
 
 AUTHOR
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