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.

git-svn-id: svn://svn.icms.temple.edu/lammps-ro/trunk@14950 f3b2605a-c512-4ea7-a41b-209d697bcdaa