update docs to add morse in a similar style to fix wall/region

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

one or more face/arg pairs may be appended :l

face = {xlo} or {xhi} or {ylo} or {yhi} or {zlo} or {zhi} :l

  args = coord epsilon \[alpha\] sigma cutoff

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

    args = coord epsilon sigma cutoff

    coord = position of wall = EDGE or constant or variable

      EDGE = current lo or hi edge of simulation box

      constant = number like 0.0 or -30.0 (distance units)

      variable = "equal-style variable"_variable.html like v_x or v_wiggle

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

      epsilon can be a variable (see below)

    alpha = width factor for wall-particle interaction (1/distance units)

      [only] for {wall/morse}. alpha can be a variable (see below)

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

      sigma can be a variable (see below)

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

  args for style {morse} :l

    args = coord D_0 alpha r_0 cutoff

    coord = position of wall = EDGE or constant or variable

      EDGE = current lo or hi edge of simulation box

      constant = number like 0.0 or -30.0 (distance units)

      variable = "equal-style variable"_variable.html like v_x or v_wiggle

    D_0 = depth of the potential (energy units)

      D_0 can be a variable (see below)

    alpha = width factor for wall-particle interaction (1/distance units)

      alpha can be a variable (see below)

    r_0 = distance of the potential minimum from the face of region (distance units)

      r_0 can be a variable (see below)

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

zero or more keyword/value pairs may be appended :l

keyword = {units} or {fld} :l

  {units} value = {lattice} or {box}

spring is at the {cutoff}.  This is a repulsive-only spring since the

interaction is truncated at the {cutoff}

For the {wall/morse} style, one additional parameter {alpha} is required.

Thus the parameters are in this order: {epsilon} as the depth of the

Morse potential (D_0), {alpha} as the width parameter of the Morse

potential, and {sigma} the location of the minimum (r_0)

the wall.  {D_0} has energy units, {alpha} inverse distance units, and

{r_0} distance units.

For the {wall/morse} style, the three parameters are in this order:

{D_0} the depth of the potential, {alpha} the width parameter, and

{r_0} the location of the minimum.  {D_0} has energy units, {alpha}

inverse distance units, and {r_0} distance units.

For any wall, the {epsilon} and/or {sigma} and/or {alpha} parameter can

be specified