Add docs on ProofLimit

frame/atomic-swap/src/lib.rs

@@ -69,6 +69,15 @@ pub trait Trait: frame_system::Trait {
  /// The currency mechanism.
  type Currency: ReservableCurrency<Self::AccountId>;
  /// Limit of proof size.
+ ///
+ /// Atomic swap is only atomic if once the proof is revealed, both parties can submit the proofs
+ /// on-chain. If A is the one that generates the proof, then it requires that either:
+ /// - A's blockchain has the same proof length limit as B's blockchain.
+ /// - Or A's blockchain has shorter proof length limit as B's blockchain.
+ ///
+ /// If B sees A is on a blockchain with larger proof length limit, then it should kindly refuse
+ /// to accept the atomic swap request if A generates the proof, and asks that B generates the
+ /// proof instead.
  type ProofLimit: Get<u32>;
  /// Block when the swap completely expires.
  type ExpireDuration: Get<BlockNumberFor<Self>>;