[SPARK-49750][DOC] Mention delegation token support in K8s mode

### What changes were proposed in this pull request?

Update docs to mention delegation token support in K8s mode.

### Why are the changes needed?

The delegation token support in K8s mode has been implemented since 3.0.0 via SPARK-23257.

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

Yes, docs are updated.

### How was this patch tested?

Review.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#48199 from pan3793/SPARK-49750.

Authored-by: Cheng Pan <chengpan@apache.org>
Signed-off-by: Dongjoon Hyun <dongjoon@apache.org>

docs/security.md

@@ -947,7 +947,7 @@ mechanism (see `java.util.ServiceLoader`). Implementations of
 `org.apache.spark.security.HadoopDelegationTokenProvider` can be made available to Spark
 by listing their names in the corresponding file in the jar's `META-INF/services` directory.
 
-Delegation token support is currently only supported in YARN mode. Consult the
+Delegation token support is currently only supported in YARN and Kubernetes mode. Consult the
 deployment-specific page for more information.
 
 The following options provides finer-grained control for this feature: