Documentation for the User field is missing
The user field has been in Acf since version 5, Please add the documentation.
Yep, I don’t think it has ever existed. I can’t add it but I can give you some information on the field.
The user field returns either an user object that is the same as what is returned by get_userdata() https://codex.wordpress.org/Function_Reference/get_userdata
Or it returns an array that is constructed by using that information
$value = array(
'ID' => $user_id,
'user_firstname' => $wp_user->user_firstname,
'user_lastname' => $wp_user->user_lastname,
'nickname' => $wp_user->nickname,
'user_nicename' => $wp_user->user_nicename,
'display_name' => $wp_user->display_name,
'user_email' => $wp_user->user_email,
'user_url' => $wp_user->user_url,
'user_registered' => $wp_user->user_registered,
'user_description' => $wp_user->user_description,
'user_avatar' => get_avatar( $user_id ),
And it returns either a single user or an array of users depending on the “Multiple” setting.