docs: Add class documentation to ResultTypeConstantsCopy

Author: TimothyJones

src/main/java/io/contract_testing/contractcase/client/ResultTypeConstantsCopy.java

@@ -4,6 +4,9 @@
 import io.contract_testing.contractcase.case_boundary.BoundaryResultTypeConstants;
 import org.jetbrains.annotations.NotNull;
 
+/**
+ * These copies exist because otherwise they can't be used as constants
+ */
 class ResultTypeConstantsCopy {
 
   /**