doc: Deprecation reason of settled on Invoice

This documents how to deal with `settled` being deprecated. It took me quite a bit of digging without such documentation.

[skip ci]

lnrpc/lightning.swagger.json

@@ -4722,6 +4722,7 @@
         },
         "settled": {
           "type": "boolean",
+          "description": "The field is deprecated. Use the state field instead (compare to SETTLED).",
           "title": "Whether this invoice has been fulfilled"
         },
         "creation_date": {