clarifications, especially on unary operators

git-svn-id: https://svn.r-project.org/R/trunk@63299 00db46b3-68df-0310-9c12-caf00c1e9a41
typo

git-svn-id: https://svn.r-project.org/R/trunk@63301 00db46b3-68df-0310-9c12-caf00c1e9a41
add regression tests fof seq() messages

git-svn-id: https://svn.r-project.org/R/trunk@63302 00db46b3-68df-0310-9c12-caf00c1e9a41
more explicit error messages

git-svn-id: https://svn.r-project.org/R/trunk@63303 00db46b3-68df-0310-9c12-caf00c1e9a41