APIドキュメントの改善 (#6757)

* Update api document in admin/announcements

* Update api document in announcements

* Update api document in i/read-announcements

* Update api document in username/available

* Update api document & Fix typo in API 403 error

* Update api document

* Update api document

* Update api document

* Fix API permission definition

* Update api document

* Update api document

* Update api document

* Update api document

* Update api document

* Update api document

* Update api document

* Update api document

* Fix bug in users (api)

* Apply reviews #6757

* Apply reviews #6757

Co-authored-by: syuilo <Syuilotan@yahoo.co.jp>