stress that code in \donttest must be valid and packages declared

git-svn-id: https://svn.r-project.org/R/trunk@67122 00db46b3-68df-0310-9c12-caf00c1e9a41
check contents of \donttest examples

git-svn-id: https://svn.r-project.org/R/trunk@67124 00db46b3-68df-0310-9c12-caf00c1e9a41