[Documentation] Express Entity - Create the entity - User Selector

[BloodyIron]

Hey so I don't have permissions to PR or whatever, so I'm just going to paste here something I found worth adding to the documentation as it is absent, but testing shows this works:

// user_selector $eObj->addAttribute('user_selector', 'Which User', 'which_user');

[shahroq]

Thank you. I appreciate it if you fork and send a pull request on this.