Creating a custom shortcode is not really an ACF subject, even if you’re using an ACF field to do it, which is why you’re not going to find any information on it on the ACF site or in any of the documentation.
This WP page has most of the information you’ll need on creating shortcodes https://codex.wordpress.org/Function_Reference/add_shortcode and you can find many tutorials on creating custom sortcodes.
