update fix wall/region docs as requested by @sjplimp (i hope)

ID, group-ID are documented in "fix"_fix.html command :ulb,l

wall/region = style name of this fix command :l

region-ID = region whose boundary will act as wall :l

style = {lj93} or {lj126} or {lj1043} or {morse} or {colloid} or {harmonic} :l

args = :l

style = {lj93} or {lj126} or {lj1043} or {colloid} or {harmonic} or {morse} :l

args for styles {lj93} or {lj126} or {lj1043} or {colloid} or {harmonic} = :l

   epsilon = strength factor for wall-particle interaction (energy or energy/distance^2 units)

   alpha (width parameter), only if style = {morse}

   sigma = size factor for wall-particle interaction (distance units) :pre

args for style {morse} = :l

   D_0 = depth of the potential (energy units)

   alpha = width parameter (1/distance units)

   r_0 = distance of the potential minimum from wall position (distance units) :pre

cutoff = distance from wall at which wall-particle interaction is cut off (distance units) :l

:ule