changes to the energy only consider the kinetic energy, so make that explicit in the augmented `fix dt/reset` docs

Allow limiting of the maximum energy change of an atom in
fix dt/reset in addition to the existing distance limit.
Useful especially for high-energy irradiation.

this introduces an optional keyword, so backward compatibility is preserved.
this closes #884

provide keyword/value option to compute ackland/atom for selecting legacy or current variant of implementation