clarify what zero standard deviation means

git-svn-id: https://svn.r-project.org/R/trunk@65950 00db46b3-68df-0310-9c12-caf00c1e9a41
for consistency with [dpq]norm

git-svn-id: https://svn.r-project.org/R/trunk@65952 00db46b3-68df-0310-9c12-caf00c1e9a41