[improve][doc] Java example explaining code without using lombok (#19019

)

site2/docs/schema-understand.md

@@ -122,15 +122,25 @@ Pulsar gets the schema definition from the predefined `struct` using an Avro lib
 1. Create the _User_ class to define the messages sent to Pulsar topics.
 
    ```java
+   # If you use Lombok
+
+   @Builder
+   @AllArgsConstructor
+   @NoArgsConstructor
    public static class User {
        public String name;
        public int age;
-       public User(String name, int age) {
- 	this.name = name;
-	this.age = age
-       }
-       public User() {}
    }
+
+   # If you DON'T use Lombok you will need to add the constructor like this
+   # 
+   #   public static class User {
+   #    String name;
+   #    int age;
+   #    public User() { } 
+   #    public User(String name, int age) { this.name = name; this.age = age; } }
+   #}
+
    ```
 
 2. Create a producer with a `struct` schema and send messages.
@@ -363,4 +373,4 @@ The following table outlines the mapping between the schema compatibility check
 |  `ALWAYS_INCOMPATIBLE`  |   N/A  | The schema evolution is disabled.                                                                                                                                                                                                                                                                                   | 
 |  <li>`BACKWARD` </li><li>`BACKWARD_TRANSITIVE` </li> |   Consumer first  | There is no guarantee that consumers using the old schema can read data produced using the new schema. Consequently, **upgrade all consumers first**, and then start producing new data.                                                                                                                            | 
 |  <li>`FORWARD` </li><li>`FORWARD_TRANSITIVE` </li> |   Producer first  | There is no guarantee that consumers using the new schema can read data produced using the old schema. Consequently, **upgrade all producers first** to use the new schema and ensure the data that has already been produced using the old schemas are not available to consumers, and then upgrade the consumers. | 
-|  <li>`FULL` </li><li>`FULL_TRANSITIVE` </li> |   Any order  | It is guaranteed that consumers using the old schema can read data produced using the new schema and consumers using the new schema can read data produced using the old schema. Consequently, you can upgrade the producers and consumers in **any order**.                                                        |
+|  <li>`FULL` </li><li>`FULL_TRANSITIVE` </li> |   Any order  | It is guaranteed that consumers using the old schema can read data produced using the new schema and consumers using the new schema can read data produced using the old schema. Consequently, you can upgrade the producers and consumers in **any order**.                                                        |