add `lncli printmacaroon` to API docs

[ZZiigguurraatt]

lncli printmacaroon is not present in the API docs. It is mentioned in the builders guide (https://docs.lightning.engineering/lightning-network-tools/lnd/macaroons).

I think lncli printmacaroon is not automatically mentioned in the API docs because it is actually not an API. Perhaps we should add an API for it.

As a user of LND, I assume I should be able to read the API docs on the web instead of running lncli -h in a terminal to know what I can do with lncli.