clarifications

git-svn-id: https://svn.r-project.org/R/trunk@67003 00db46b3-68df-0310-9c12-caf00c1e9a41
repeat comment in footnote

git-svn-id: https://svn.r-project.org/R/trunk@67004 00db46b3-68df-0310-9c12-caf00c1e9a41