g_get_tmp_dir(): Clean up envars

On UNIX, we should only ever be looking at TMPDIR.

On Windows, we should only ever look at TEMP.

Also, clean up the documentation to better describe what is actually
happening.  The previous docs may have left someone confused about why
this function returns "/var/tmp" on Solaris, even with no TMPDIR set.

https://bugzilla.gnome.org/show_bug.cgi?id=705075