pkgctl auth: document and improve

The manpage for pkgctl auth is referenced here but apparrently not existent. Did it just never get commited or is it not created at all?

Also the behaviour introduced in !135 (merged) is not really documented as a consquence.

While one is at it an additional improvement would be to make the user aware of the choice between saving the token in a file or supplying it via DEVTOOLS_GITLAB_TOKEN while setting things up here: https://gitlab.archlinux.org/archlinux/devtools/-/blob/git-packages/src/lib/auth/login.sh#L92