Documentation is outdated/wrong

Some minor issues in POD:

File::XDG supports `XDG_RUNTIME_DIR`, [but it says otherwise](https://github.com/uperl/File-XDG/blob/main/lib/File/XDG.pm#L37-L39).

[Displays old XDG spec version](https://github.com/uperl/File-XDG/blob/main/lib/File/XDG.pm#L547); latest is 0.8.

[Path::Spec instead of File::Spec](https://github.com/uperl/File-XDG/blob/main/lib/File/XDG.pm#L581).