Update docs on converting pfsArm fluxes to counts

datamodel.txt states:

 
Note that the pfsFiberTrace file contains enough information to recover the raw counts
from the arbitrary units used for the flux, and the widths of the fiber traces.

I believe that this now means pfsFiberProfile (the word appears in the file twice), and this should be updated.

In addition, please provide a few more details. I think that it means "multiply by fiberProfiles[fiberId].norm".