Documentation: exclude media documentation from pdf generation

Although pdflatex is more robust than rst2pdf, building media
documentation pdf still fails. Exclude media documentation from pdf
generation for now.

Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Acked-by: Markus Heiser <markus.heiser@darmarit.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>