message: Add explanatory comment.
diff: @@ -98,6 +98,9 @@ impl LinearChain {
  }
  // Cache the signatures as we expect more finality signatures for the new block to
  // arrive soon.
  + // If `signatures` was empty, it will serve as a flag that the `block` is already
  + // finalized/known and any incoming signatures should be accepted (given they're
  + // from a bonded validator and valid).
  self.cache_signatures(block_signatures);
  signatures
  }
  