Add comment to ProofOfWork.payload about the recent changes and futur…

…e rename.

security/src/main/java/bisq/security/pow/ProofOfWork.java

@@ -35,8 +35,13 @@
 @Getter
 @EqualsAndHashCode
 public final class ProofOfWork implements NetworkProto {
-    // payload is usually the pubKeyHash
-    private final byte[] payload;       // message of 1000 chars has about 1300 bytes
+    // For HashCashTokens we used the serialized message data, thus it was called payload, and
+    // it could become quite large (e.g. inventory request with 2MB size limit)
+    // We fixed that design mistake with HashCashV2Token and using the hash of the protobuf data.
+    // With that we will get a fixes size of 20 bytes. Once the HashCashToken is not used anymore we can rename
+    // the field to hash and change the verify methods to expect 20 bytes.
+    private final byte[] payload;
+
     private final long counter;
     // If challenge does not make sense we set it null
     // Challenge need to be hashed to 256 bits