chore(java): add fury deep copy documentation (#1773)

## What does this PR do?

add fury deep copy documentation

## Related issues

Closes #1772

## Does this PR introduce any user-facing change?

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- [ ] Does this PR introduce any public API change?
- [ ] Does this PR introduce any binary protocol compatibility change?


## Benchmark

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docs/guide/java_serialization_guide.md

@@ -79,10 +79,10 @@ import org.apache.fury.config.*;
 public class Example {
  // reuse fury.
  private static final ThreadSafeFury fury = new ThreadLocalFury(classLoader -> {
-  Fury f = Fury.builder().withLanguage(Language.JAVA)
-  .withClassLoader(classLoader).build();
-  f.register(SomeClass.class);
-  return f;
+ Fury f = Fury.builder().withLanguage(Language.JAVA)
+ .withClassLoader(classLoader).build();
+ f.register(SomeClass.class);
+ return f;
  });
 
  public static void main(String[] args) {
@@ -115,6 +115,7 @@ public class Example {
 | `codeGenEnabled` | Disabling may result in faster initial serialization but slower subsequent serializations. | `true` |
 | `asyncCompilationEnabled` | If enabled, serialization uses interpreter mode first and switches to JIT serialization after async serializer JIT for a class is finished. | `false` |
 | `scalaOptimizationEnabled` | Enables or disables Scala-specific serialization optimization. | `false` |
+| `copyRef` | When disabled, the copy performance will be better. But fury deep copy will ignore circular and shared reference. Same reference of an object graph will be copied into different objects in one `Fury#copy`. | `true` |
 
 ## Advanced Usage
 
@@ -192,6 +193,29 @@ not worthy compared to performance cost. Maybe you should try to disable long co
 much
 space savings.
 
+### Object deep copy
+
+Deep copy example:
+
+```java
+Fury fury=Fury.builder()
+ ...
+ .withRefCopy(true).build();
+ SomeClass a=xxx;
+ SomeClass copied=fury.copy(a)
+```
+
+Make fury deep copy ignore circular and shared reference, this deep copy mode will ignore circular and shared reference.
+Same reference of an object graph will be copied into different objects in one `Fury#copy`.
+
+```java
+Fury fury=Fury.builder()
+ ...
+ .withRefCopy(false).build();
+ SomeClass a=xxx;
+ SomeClass copied=fury.copy(a)
+```
+
 ### Implement a customized serializer
 
 In some cases, you may want to implement a serializer for your type, especially some class customize serialization by