Document solids.momentum.update and solids.energy.update

Documents solids.momentum.update and solids.energy.update (exa/mfix#313) in place of solids.enable.momentum, solids.enable.energy and dem.implicit_drag, with a table of defaults and invalid combinations for DEM, PIC, virtual mass and radiation. Companion to the exa/mfix solids-update-type MR; merge them together, because the GUI key data is generated from this text.