Add comment to XContentBuilder.toString() (opensearch-project#13809) (o…

…pensearch-project#13932)

* Add comment to XContentBuilder.toString

* Fix typo

---------

(cherry picked from commit bf43678)

Signed-off-by: Liyun Xiu <xiliyun@amazon.com>
Signed-off-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>
Signed-off-by: kkewwei <kkewwei@163.com>

libs/core/src/main/java/org/opensearch/core/xcontent/XContentBuilder.java

@@ -157,6 +157,9 @@ public static XContentBuilder builder(XContent xContent, Set<String> includes, S
 
     /**
      * Returns a string representation of the builder (only applicable for text based xcontent).
+     * Note: explicitly or implicitly (from debugger) calling toString() could cause XContentBuilder
+     * to close which is a side effect done by @see BytesReference#bytes().
+     * Trying to write more contents after toString() will cause NPE. Use it with caution.
      */
     @Override
     public String toString() {