Peer Discovery: document HTTPS seeding

Reading https://github.com/decred/dcrd/pull/2188 I'm getting the impression that there are two seeding methods in dcrd now, DNS and HTTPS. If that is correct, please add information about HTTPS seeding to the [Peer Discovery](https://devdocs.decred.org/protocol-p2p-network/peer-discovery/) page.