Allow value of assembly.link_reference_style to be non-absolute when generating PDF

When the assembly.link_reference_style key was added, the thinking was that it wouldn't make sense for PDF. That's because, once a PDF is downloaded (or otherwise shared), relative references to other resources break since they won't come along with the PDF. However, this is really a user choice. The user may ship the site as an archive, in which case the relative references will still be resolvable.

Lift the restriction of using values other than absolute when generating PDF. (If we find that other formats are needed in the future, we can revisit the restriction once again).