Documentation for capacity-limit-backup-calculator and capacity-limit values/functions

[robbieorvis]

I'm trying to better understand how the default values for backup capacity were determined for GCAM and how users can change these. I know that that the capacity limit can be changed directly via the XML files and that insert a value can modify the curvature, but I'm hoping to find the equations for this function so I can better modify the function. The docs say it is documented in the electricity section but it's not really discussed and there are no equations.