doc: for ps to pdf conversion, use "acrodist", "ps2pdf" or "pstopdf"

doc: for ps to pdf conversion, use "acrodist", "ps2pdf" or "pstopdf"

For PostScript to PDF conversion, use whichever of "acrodist",
"ps2pdf", or "pstopdf" which we can find on the system.  I haven't
tried either acrodist or pstopdf myself, only going my the
documentation, but prefer acrodist since it is claimed to produce
smaller output files than ps2pdf.