Add experimental warning

Author: waahm7

Diff for: include/aws/s3/s3_client.h

@@ -552,6 +552,9 @@ struct aws_s3_client_config {
      * If set, client will invoke the factory to get the provider to use, when needed.
      *
      * If not set, client will create a default S3 Express provider under the hood.
+     *
+     * NOTE: THE FOLLOWING BEHAVIOR IS EXPERIMENTAL AND UNSTABLE
+     * Default S3 Express provider will pass the headers allowed in `g_s3_create_session_allowed_headers` to the CreateSession call.
      */
     aws_s3express_provider_factory_fn *s3express_provider_override_factory;
     void *factory_user_data;