Add Javadoc for getUnsafeNativeHandle

Signed-off-by: Ryan Nett <JNett96@gmail.com>

Author: rnett

tensorflow-core/tensorflow-core-api/src/main/java/org/tensorflow/GraphOperation.java

@@ -383,6 +383,11 @@ public Set<GraphOperation> controlConsumers() {
     }
   }
 
+  /**
+   * Get the native handle of this operation.
+   *
+   * No liveness or non-null checking is done, the operation may have been deallocated.
+   */
   public TF_Operation getUnsafeNativeHandle() {
     return unsafeNativeHandle;
   }