Update set command documentation

doc/src/set.txt

@@ -36,6 +36,8 @@ keyword = {type} or {type/fraction} or {mol} or {x} or {y} or {z} or \
     value can be an atom-style variable (see below)
   {x},{y},{z} value = atom coordinate (distance units)
     value can be an atom-style variable (see below)
+  {vx},{vy},{vz} value = atom velocity (velocity units)
+    value can be an atom-style variable (see below)
   {charge} value = atomic charge (charge units)
     value can be an atom-style variable (see below)
   {dipole} values = x y z
@@ -127,6 +129,7 @@ set type 3 charge 0.5
 set type 1*3 charge 0.5
 set atom * charge v_atomfile
 set atom 100*200 x 0.5 y 1.0
+set atom 100 vx 0.0 vy 0.0 vz -1.0
 set atom 1492 type 3 :pre
 
 [Description:]
@@ -225,7 +228,8 @@ IDs.
 
 Keywords {x}, {y}, {z}, and {charge} set the coordinates or charge of
 all selected atoms.  For {charge}, the "atom style"_atom_style.html
-being used must support the use of atomic charge.
+being used must support the use of atomic charge. Keywords {vx}, {vy},
+and {vz} set the velocities of all selected atoms.
 
 Keyword {dipole} uses the specified x,y,z values as components of a
 vector to set as the orientation of the dipole moment vectors of the