Properly split and document the `admin_post*` actions into the following four hooks:

* `admin_post_nopriv` – for logged-out requests lacking a supplied action
* `admin_post_nopriv_$action – for logged-out requests with a supplied action
* `admin_post` – for logged-in requests lacking a supplied action
* `admin_post_$action` – for logged-in requests with a supplied action

See [28349], [28350], [28351].
See #26869.


git-svn-id: https://develop.svn.wordpress.org/trunk@28394 602fd350-edb4-49c9-b593-d223f7449a82