[docs] Review: consensus

[semenov-vladyslav]

Doc page consensus contains the following defects:

• consensus doc is pretty shallow, maybe expand a little bit on the protocol and its features
• On IOTA, consensus has a simple API paragraph -- confusing description of the consensus protocol.
  • What follows after API is not an API. A better way to write that would be: "Users submit transactions and in a while receive a transaction finalization confirmation." Or, "Validators accept transactions, run consensus protocol and produce consistently ordered stream of transactions." But it's common for all blockchains. What specific to IOTA consensus protocol is not clear at all.
  • validators submit -- users/clients submit txs to validators. Validators can't submit anything to consensus, consensus is a state among validators in wider sense or a protocol in narrow sense.
  • the consensus outputs -- again, consensus can't output anything.
  • consistent stream of transactions -- readers might think "consistent" here means "with fixed rate", what actually meant is "transactions are consistently ordered among (honest) majority of validators". Just use clearer terminology.
  • well-behaving validators -- How to identify such "well-behaving" validators? What if these "well-behaving" validators are minority?
• **Mysticeti** protocol -- put a link to the paper right away. Add a brief description of the protocol and mention that it is DAG-based (eg. "Mysticeti is a leaderless BFT protocol with optimized commit rule designed for low-latency and high-throughput utilizing an uncertified DAG structure", and in the list below we can explain what these specific terms mean and why they matter), otherwise "DAG-based protocol" term is used later without any reference to Mysticeti.
• leaders on blocks -- undefined term which is not used anywhere else in the docs. Either explain it, or use synonym.
• It's also good to mention here that IOTA uses delegated PoS consensus.
• Consensus on IOTA section in about-iota talks about delegated PoS. Consensus on IOTA titled consensus page talks about Mysticeti. It's confusing al least. So, what is the "Consensus on IOTA"? Is it delegated PoS or Mysticeti?