add gravity keyword info to fix rigid doc page

doc/src/fix_rigid.rst

@@ -610,10 +610,12 @@ such attributes: the total mass of the rigid body, its center-of-mass
 position, its 6 moments of inertia, its center-of-mass velocity, and
 the 3 image flags of the center-of-mass position.  For rigid bodies
 consisting of point particles or non-overlapping finite-size
-particles, LAMMPS can compute these values accurately.  However, for
-rigid bodies consisting of finite-size particles which overlap each
-other, LAMMPS will ignore the overlaps when computing these 4
-attributes.  The amount of error this induces depends on the amount of
+particles, LAMMPS can compute these values accurately.
+
+However, for rigid bodies consisting of finite-size particles which
+overlap each other, LAMMPS will ignore the overlaps when computing
+these 4 attributes, which means the dynamics of the bodies will be
+incorrect.  The amount of error this induces depends on the amount of
 overlap.  To avoid this issue, the values can be pre-computed
 (e.g. using Monte Carlo integration).
 
@@ -677,6 +679,28 @@ cross periodic boundaries during the simulation.
    auxiliary file will contain one line for every rigid body, even if the
    original file only listed a subset of the rigid bodies.
 
+If the system has rigid bodies with finite-size overlapping particles
+and the model uses the :doc:`fix gravity <fix_gravity>` command to
+apply a gravitational force to the rigid bodies, then the *gravity*
+keyword should be used in the following manner.
+
+First, the group specified for the :doc:`fix gravity <fix_gravity>`
+command should not include any atoms in rigid bodies which have
+overlapping particles.  It can be empty (see the :doc:`group empty
+<group>` command) or only contain single particles not in rigid
+bodies, e.g. background particles.
+
+Second, the *infile* keyword should be used to specify the total mass
+and other properties of the rigid bodies with overlaps, so that their
+dyamics will be modeled correctly, as explained above.
+
+Third, the *gravity* keyword should be used the with the ID of the
+:doc:`fix gravity <fix_gravity>` command as its argument.  The rigid
+fixes will access gravity fix to extract the current direction of the
+gravity vector at each timestep (which can be static or dynamic).  A
+gravity force will then be applied to each rigid body at its
+center-of-mass position using its total mass.
+
 ----------
 
 If you use a :doc:`temperature compute <compute>` with a group that