Restrict Admin search to Admin ?

No, it’s not. The admin search is, by default, available to everyone with the edit_posts capability, which includes contributors. It doesn’t grant anyone any access to something they otherwise wouldn’t be able to access.

If you want to restrict the admin search, you can use the relevanssi_admin_search_capability filter hook to change the required capability. If you want to restrict it to only authors, use publish_posts; to editors, use edit_others_posts.

add_filter( 'relevanssi_admin_search_capability', function( $cap ) {
  return 'edit_others_posts';
} );

Hi, sorry for the long delay in my answer, but apparently didn’t wordpress send me any notification..

I’m pretty new at wordpress, it is enough to add that string to functions.php to prevent relevanssi ADMIN search for everybody except admin?

Yes, add it to theme functions.php or in a code snippet plugin.