How to Set Taxonomy Object Description?

I don’t see an option for it in register_taxonomy()

It’s not in Codex, but is fully supported:

add_action( 'init', function() {

    register_taxonomy(
      'foo',
      'post',       
      array(
        'description' => 'Hi there!'
      )
    );

    $foo = get_taxonomy( 'foo' );
    echo $foo->description; // output "Hi there!"

});