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.
With this change we allow adding hop hints when adding an invoice, even
if its amount is zero.
A couple of new unit test case have been added, and the
`testInvoiceRoutingHints` itest was expanded to account for this
scenario.
alpeb-btc