Mainly frees memory used by the document. Also closes a file, if
it was not closed before with PS_close(3). You should in any case close
the file with PS_close(3) before, because PS_close(3) not just closes the
file but also outputs a last line containing the PostScript comment with
the number of pages in the document. Without this comment, a PostScript
viewer may not show the number of pages propperly.