Remove descriptor lookup section and incorporate a few sentences and
the citation from that section into an earlier section where we first
talk about the descriptorpool environment, per Jeroen's comment.



git-svn-id: svn://svn.r-forge.r-project.org/svnroot/rprotobuf@771 edb9625f-4e0d-4859-8d74-9fd3b1da38cb