Update inputs page telling how the inputs are used to set the timestep.

docs/source/InputsTimeStepping.rst

@@ -3,7 +3,11 @@
 Time Stepping
 =============
 
-The following inputs must be preceded by "amr."
+The following inputs must be preceded by "amr."   Note that if both are specified, both criteria
+are used and the simulation still stop when the first criterion is hit.  In the case of unsteady flow,
+the simulation will stop when either the number of steps reaches max_step or time reaches stop_time.
+In the case of unsteady flow, the simulation will stop when either the tolerance (difference between
+subsequent steps) is reached or the number of iterations reaches the maximum number specified.
 
 +------------------+-----------------------------------------------------------------------+-------------+-----------+
 |                  | Description                                                           |   Type      | Default   |
@@ -32,8 +36,40 @@ Currently, the criterion for setting "steady_state" to true is if "dt" is undefi
 
 +-----------------------+-----------------------------------------------------------------------+-------------+------------+
 |                       | Description                                                           |   Type      | Default    |
-+======================-+=======================================================================+=============+============+
++=======================+=======================================================================+=============+============+
 | steady_state_tol      | Tolerance for checking if we have reached steady state                |   Real      | 1.e-5      |
 +-----------------------+-----------------------------------------------------------------------+-------------+------------+
 | steady_state_max_iter | Maximum number of allowed iterations to converge to steady state      |   Int       | 100000000  |
 +-----------------------+-----------------------------------------------------------------------+-------------+------------+
+
+
+Setting the Time Step 
+=====================
+
+There are several ways that the inputs are used to determine what time step 
+is used in the evolution of the fluid-particle system in MFiX-Exa.   
+
+1) In a pure particle case, the "mfix.fixed_dt", if specified, is only used to determine the frequency
+of outputs, it has no effect on the dt used in the particle evaluation.  [WHAT HAPPENS IF FIXED_DT 
+NOT SPECIFIED?]
+
+That is determined by the following calculation:
+
+2) In a pure fluid case, there are two options:
+
+  * If you want to fix the dt, simply set :cpp:`mfix.fixed_dt = XXX` and the fluid time
+    step will always be that number.  [SHOULD WE ABORT IF YOU ALSO SET CFL?  OR SPECIFY
+    THAT FIXED_DT OVERRIDES CFL?]
+
+  * If you want to let the code determine the appropriate time step using the advective CFL
+    condition, then set :cpp:`mfix.cfl = 0.5` for example, and the fluid time step will
+    be computed to be dt = 0.5 * dx / max(vel).
+
+Note that these choices apply to steady state calculations as well as unsteady runs.
+
+
+3) In a coupled particle-fluid case, dt is set as in the pure-fluid case.  In this case
+   the particle time step is first computed as in the particle-only case, then is adjusted
+   so that an integral number of particle steps fit into a single fluid time step.
+
+[HOW ARE DT_MIN AND DT_MAX USED???]