Revert "Revert "javadoc""

This reverts commit 2c7c292.

xrpl4j-core/src/main/java/org/xrpl/xrpl4j/model/transactions/UnknownTransaction.java

@@ -8,6 +8,9 @@
 import org.immutables.value.Value.Immutable;
 import org.xrpl.xrpl4j.model.flags.TransactionFlags;
 
+/**
+ * Mapping for any transaction type that is unrecognized/unsupported by xrpl4j.
+ */
 @Immutable
 @JsonSerialize(as = ImmutableUnknownTransaction.class)
 @JsonDeserialize(as = ImmutableUnknownTransaction.class)
@@ -23,7 +26,9 @@ static ImmutableUnknownTransaction.Builder builder() {
   }
 
   /**
-   * This has to be a {@link String} because {@link Transaction#transactionType()} is a {@link TransactionType},
+   * The actual transaction type found in the {@code "TransactionType"} field of the transaction JSON.
+   *
+   * <p>This has to be a {@link String} because {@link Transaction#transactionType()} is a {@link TransactionType},
    * which only has an UNKNOWN variant. Because this method is also annotated with {@link JsonProperty} of
    * "TransactionType", this essentially overrides the "TransactionType" field in JSON, but {@link #transactionType()}
    * will always be {@link TransactionType#UNKNOWN} and this field will contain the actual "TransactionType" field.
@@ -33,12 +38,24 @@ static ImmutableUnknownTransaction.Builder builder() {
   @JsonProperty("TransactionType")
   String unknownTransactionType();
 
+  /**
+   * The {@link TransactionType} of this UnknownTransaction, which will always be {@link TransactionType#UNKNOWN}.
+   * {@link #unknownTransactionType()} contains the actual transaction type value.
+   *
+   * @return {@link TransactionType#UNKNOWN}.
+   */
   @Override
   @JsonIgnore
+  @Value.Derived
   default TransactionType transactionType() {
     return Transaction.super.transactionType();
   }
 
+  /**
+   * A set of {@link TransactionFlags}.
+   *
+   * @return A {@link TransactionFlags}.
+   */
   @JsonProperty("Flags")
   @Value.Default
   default TransactionFlags flags() {