Add tip on host settings for Spark provider (#33233)

docs/apache-airflow-providers-apache-spark/connections/spark.rst

@@ -56,3 +56,10 @@ For example:
 .. code-block:: bash
 
    export AIRFLOW_CONN_SPARK_DEFAULT='spark://mysparkcluster.com:80?deploy-mode=cluster&spark_binary=command&namespace=kube+namespace'
+
+.. warning::
+
+  Make sure you trust your users with the ability to configure the host settings as it may enable the connection to
+  establish communication with external servers. It's crucial to understand that directing the connection towards a
+  malicious server can lead to significant security vulnerabilities, including the risk of encountering
+  Remote Code Execution (RCE) attacks.