That button does not help me. I tried to trigger doc–rebuild for a few versions of
`transient` package and docs are still unavailable (for latest version I've uploaded docs manually).
TL;DR:
There is now "Maintainer's Corner" (it is located at the bottom of package page and you'll see it if you're in maintainer group) where you can find "edit package information" link. If you'll follow it you'll see maintainer's page of package where you can find "Manage documentation" section. There goes a list of uploaded package version. Clicking on arbitrary version will bring you to page with two forms, one for uploading docs and another contains mentioned "Delete documentation and trigger rebuild" button.