Greatly expand documentation comment at head of file... I had to

Greatly expand documentation comment at head of file... I had to
 try to understand it; maybe this will help someone else do so too.
Default CVSRootDir to $CVSROOT first, then the path in ~vadve only if $CVSROOT
 was not set.
Checkout with -z3 if we might be checking out over ssh/rsh.
Check the filename of gnuplot to see if it is executable; if not, as a
 last resort, try just plain "gnuplot", hoping it is in the path somewhere.
Remove a little extra whitespace.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@9046 91177308-0d34-0410-b5e6-96231b3b80d8