Outdated documentation for charged particle differential fluence calculations with cavity

[mainegra]

The PIRS-898 manual refers to these calculations using the stopping power at the bottom of the beam which is not what is currently implemented. This was the initial implementation but was changed to a series expansion of the expression for stopping power around the energy at the middle of each bin.

There is also several typos in the indices of the equations! Documentation actually was never completed. There is a PIRS report in preparation which will properly describe this implementation!

This should be updated ASAP!