pkgctl auth: document and improve
The manpage for `pkgctl auth` is referenced [here](https://gitlab.archlinux.org/archlinux/devtools/-/blob/git-packages/doc/man/pkgctl.1.asciidoc?plain=1#L53) but apparrently not existent. Did it just never get commited or is it not created at all?
Also the behaviour introduced in https://gitlab.archlinux.org/archlinux/devtools/-/merge_requests/135 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
issue