LUCENE-9424: add a performance warning to AttributeSource.captureState javadocs

lucene/CHANGES.txt

@@ -183,6 +183,12 @@ Bug Fixes
 ---------------------
 (No changes)
 
+Documentation
+---------------------
+
+* LUCENE-9424: Add a performance warning to AttributeSource.captureState javadocs (Haoyu Zhai)
+
+
 Other
 ---------------------
 (No changes)

lucene/core/src/java/org/apache/lucene/util/AttributeSource.java

@@ -292,6 +292,8 @@ public class AttributeSource {
   /**
    * Captures the state of all Attributes. The return value can be passed to
    * {@link #restoreState} to restore the state of this or another AttributeSource.
+   * 
+   * Be careful, this method comes with a cost of deep copying all attributes in the source.
    */
   public final State captureState() {
     final State state = this.getCurrentState();