lnrpc+cmd: clarify docs about addinvoice no-amount hop hints

Add additional comment to `lnclic addinvoice`'s `--private` flag warning
about including a large number of private channel hints when using zero
ammounts.

Also added a comment to the `private` field in the `Invoice` proto
message.

cmd/lncli/cmd_invoice.go

@@ -64,7 +64,10 @@ var addInvoiceCommand = cli.Command{
 			Name: "private",
 			Usage: "encode routing hints in the invoice with " +
 				"private channels in order to assist the " +
-				"payer in reaching you",
+				"payer in reaching you. If amt and amt_msat " +
+				"are zero, a large number of hints with " +
+				"these channels can be included, which " +
+				"might not be desirable.",
 		},
 		cli.BoolFlag{
 			Name: "amp",

lnrpc/invoicesrpc/invoices.swagger.json

@@ -514,7 +514,7 @@
         },
         "private": {
           "type": "boolean",
-          "description": "Whether this invoice should include routing hints for private channels."
+          "description": "Whether this invoice should include routing hints for private channels.\nNote: When enabled, if value and value_msat are zero, a large number of\nhints with these channels can be included, which might not be desirable."
         },
         "add_index": {
           "type": "string",

lnrpc/lightning.pb.go

@@ -11640,6 +11640,8 @@ type Invoice struct {
 	// invoice's destination.
 	RouteHints []*RouteHint `protobuf:"bytes,14,rep,name=route_hints,json=routeHints,proto3" json:"route_hints,omitempty"`
 	// Whether this invoice should include routing hints for private channels.
+	// Note: When enabled, if value and value_msat are zero, a large number of
+	// hints with these channels can be included, which might not be desirable.
 	Private bool `protobuf:"varint,15,opt,name=private,proto3" json:"private,omitempty"`
 	// The "add" index of this invoice. Each newly created invoice will increment
 	// this index making it monotonically increasing. Callers to the

lnrpc/lightning.proto

@@ -3401,6 +3401,8 @@ message Invoice {
     repeated RouteHint route_hints = 14;
 
     // Whether this invoice should include routing hints for private channels.
+    // Note: When enabled, if value and value_msat are zero, a large number of
+    // hints with these channels can be included, which might not be desirable.
     bool private = 15;
 
     /*

lnrpc/lightning.swagger.json

@@ -5048,7 +5048,7 @@
         },
         "private": {
           "type": "boolean",
-          "description": "Whether this invoice should include routing hints for private channels."
+          "description": "Whether this invoice should include routing hints for private channels.\nNote: When enabled, if value and value_msat are zero, a large number of\nhints with these channels can be included, which might not be desirable."
         },
         "add_index": {
           "type": "string",