reinforce comment about Enhances

git-svn-id: https://svn.r-project.org/R/trunk@66752 00db46b3-68df-0310-9c12-caf00c1e9a41
correct section hierarchy

git-svn-id: https://svn.r-project.org/R/trunk@66754 00db46b3-68df-0310-9c12-caf00c1e9a41