update docs for pair style gauss/cut to document optional per pair cutoff

doc/src/pair_gauss.txt

@@ -63,7 +63,7 @@ solvent simulations of salt ions "(Lenart)"_#Lenart and of surfactants
 "(Jusufi)"_#Jusufi.  In these instances the Gaussian potential mimics
 the hydration barrier between a pair of particles. The hydration
 barrier is located at r_mh and has a width of sigma_h. The prefactor
-determines the hight of the potential barrier.
+determines the height of the potential barrier.
 
 The following coefficients must be defined for each pair of atom types
 via the "pair_coeff"_pair_coeff.html command as in the example above,
@@ -73,9 +73,11 @@ commands:
 
 H (energy * distance units)
 r_mh (distance units)
-sigma_h (distance units) :ul
+sigma_h (distance units)
+cutoff (distance units) :ul
 
-The global cutoff (r_c) specified in the pair_style command is used.
+The last coefficient is optional. If not specified, the global cutoff
+is used.
 
 :line