message: Extend Horizon doc comment.
diff: @@ -11,6 +11,12 @@ use crate::components::consensus::{
  type Committee = Vec<ValidatorIndex>;
  /// A list containing the earliest level-n messages of each member of some committee, for some n.
  +///
  +/// A summit is a sequence of committees of validators, where each member of the level-n committee
  +/// has produced a vote that can see level-(n-1) votes by a quorum of the level-n committe.
  +///
  +/// The level-n horizon maps each validator of a level-n committee to their earliest level-n vote.
  +/// From a level-n horizon, the level-(n+1) committee and horizon can be computed.
  #[derive(Debug)]
  pub(super) struct Horizon<'a, C: Context> {
  /// Assigns to each member of a committee the sequence number of the earliest message that
  