Corrected Point-Of-Sale HID descriptor example 6.1.6

Also made minor cosmetic changes.
Added unicode superscripts for UNIT_EXPONENT decodes
Reworked UNIT and UNIT_EXPONENT handling.

The formula used to convert logical units into physical units is
now displayed as a comment next to each field in the C structure
output. Instead of just displaying the formula verbatim, an attempt
is made to reduce it to a simpler form. For example, instead of:
  physical = (logical - 0) x 256000/256 + 0
the formula is reduced to:
  physical = logical x 1000
It's not perfect but better than the former.