reword implementation recommendations

Author: devrandom

lightning/src/chain/keysinterface.rs

@@ -201,9 +201,8 @@ pub trait ChannelKeys : Send+Clone {
 	fn get_per_commitment_point<T: secp256k1::Signing + secp256k1::Verification>(&self, idx: u64, secp_ctx: &Secp256k1<T>) -> PublicKey;
 	/// Gets the commitment secret for a specific commitment number as part of the revocation process
 	///
-	/// An implementation must error here if the commitment was already signed.
-	/// After this method is called, an implementation must ensure that it will refuse sign the relevant
-	/// local commitment transaction.
+	/// An external signer implementation should error here if the commitment was already signed
+	/// and should refuse to sign it in the future.
 	///
 	/// May be called more than once for the same index.
 	///
@@ -229,7 +228,7 @@ pub trait ChannelKeys : Send+Clone {
 	/// Create a signature for a local commitment transaction. This will only ever be called with
 	/// the same local_commitment_tx (or a copy thereof), though there are currently no guarantees
 	/// that it will not be called multiple times.
-	/// The commitment must not have been revoked.
+	/// An external signer implementation should check that the commitment has not been revoked.
 	//
 	// TODO: Document the things someone using this interface should enforce before signing.
 	// TODO: Add more input vars to enable better checking (preferably removing commitment_tx and