Update pkg-refs.Rmd to include info on PAT environment variables.

[rorynolan]

Closes r-lib/pak#588

Also fixes some small errors in the GitLab section.

[rorynolan]

I think it would also be nice to

• match up the discovered env vars with remotes (I think GITHUB_PAT and GITLAB_PAT are good)
• match up the location specification with remotes ([<package>=][github::]<username>/<repository>[/<subdir>][<detail>])