Improve docs for tempname() and tempdir()

I find that "filename" is quite confusing in the docs for `tempname`. I guess for most users "filename" means the part of the path up to the parent directory. The man page for `tempnam` doesn't say what's the definition of "filename", but I guess that's defined in POSIX. I think `path` is clearer, else you'd want to concatenate `tempdir()` and `tempname()` to get the path to a file.