Use 'vignettes' in preference to 'inst/doc' when looking for package
vignette sources.

git-svn-id: https://svn.r-project.org/R/trunk@63074 00db46b3-68df-0310-9c12-caf00c1e9a41
Ensure that required DESCRIPTION fields are neither missing nor empty.
Have CRAN incoming checks note vignette sources only in 'inst/doc'.

git-svn-id: https://svn.r-project.org/R/trunk@63075 00db46b3-68df-0310-9c12-caf00c1e9a41