Consolidate accessing CRAN objects.

git-svn-id: https://svn.r-project.org/R/trunk@66474 00db46b3-68df-0310-9c12-caf00c1e9a41
Mention doi and url fields.

git-svn-id: https://svn.r-project.org/R/trunk@66475 00db46b3-68df-0310-9c12-caf00c1e9a41
update

git-svn-id: https://svn.r-project.org/R/trunk@66476 00db46b3-68df-0310-9c12-caf00c1e9a41