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

* Add comment to XContentBuilder.toString

Signed-off-by: Liyun Xiu <xiliyun@amazon.com>

* Fix typo

Signed-off-by: Liyun Xiu <xiliyun@amazon.com>

---------

Signed-off-by: Liyun Xiu <xiliyun@amazon.com>

Author: chishui

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() {