Document overridden methods

Author: yevhenii-nadtochii

java/src/main/kotlin/io/spine/validation/java/setonce/SetOnceEnumField.kt

@@ -118,7 +118,12 @@ internal class SetOnceEnumField(
         asEnumConstant(fieldValue)
 
     /**
-     * Returns a string representation of the corresponding Java enum constant.
+     * Converts the given [fieldValue] to string for a diagnostics message.
+     *
+     * We have to override this method because within the builder, an enum constant
+     * is stored as [Int], but the [field] type is enum. This confuses the default
+     * implementation of [asString], which yields an expression to convert to [String]
+     * depending on the [field] type.
      */
     override fun asString(fieldValue: Expression<Int>): Expression<String> =
         fieldTypeClass

java/src/main/kotlin/io/spine/validation/java/setonce/SetOnceStringField.kt

@@ -148,6 +148,14 @@ internal class SetOnceStringField(
         fieldProcessing.addAfter(precondition, fieldProcessing.lBrace)
     }
 
+    /**
+     * Converts the given [fieldValue] to string for a diagnostics message.
+     *
+     * We have to override this method because within the builder, a string is stored
+     * as [StringOrByteString], but the [field] type is just [String]. This confuses
+     * the default implementation of [asString], which yields an expression to convert
+     * to [String] depending on the [field] type.
+     */
     override fun asString(fieldValue: Expression<StringOrByteString>): Expression<String> =
         MethodCall(fieldValue, "toString")
 }