[pdftex] 1px is not 1bp which seems to contradict the pdftex manual

[Karl Berry]

    Date: Sun, 26 Mar 2023 14:24:04 +0200
    To: "<pdftex at tug.org>" <pdftex at tug.org>
    ....
    For a pdftex user to revert to pdfTeX default after a change to "px"
    unit, the input "\pdfpxdimen 1.00001bp"

I (finally) changed the description of \pdfpxdimen to mention this tiny
discrepancy, and that is not *exactly* 72dpi:

  The default value of \cs{pdfpxdimen} is 1.00001bp (for historical
  reasons), corresponding to a pixel density of (a few sp off from) 72dpi.
  The \LUATEX\ default is 1bp (also a few sp off from 72dpi), so to get
  precisely the same behavior in \PDFTEX\ and \LUATEX, set
  \cs{pdfpxdimen}\texttt{=1bp}.

Best,
Karl