add some commentary for restarting sims or changing to transient

docs/src/user_guide/4_runtime_and_solvers.md

@@ -36,4 +36,14 @@ In XCALibre.jl the `run!` function is used to start a simulation, which will dis
 ```@docs; canonical=false
 run!()
 ```
----
+
+## Restarting simulations
+---
+
+It should be noted that when running a simulation with `run!`, the solution fields in the `Physics` model are mutated. Thus, running the simulation from the previous solution is simply a matter of reissuing the `run!` function. At present, this has the side effect of overwriting any existing solution files (`.vtk` or `.vtu`). Users must be aware of this behaviour.
+
+In some cases, it may be desirable to solve a problem with a steady solver and use the solution to run transient simulations. This is possible using the `change` function.
+
+```@docs; canonical=false
+XCALibre.ModelPhysics.change
+```

src/ModelPhysics/3_physics_API.jl

@@ -1,5 +1,25 @@
 export change
 
+"""
+ change(model::Physics, property, value) => updatedModel::Physics
+
+A convenience function to change properties of an exisitng `Physics` model.
+
+# Input arguments
+- `model::Physics` a `Physics` model to modify
+- `property` is a symbol specifying the property to change 
+- `value` is the new setting for the specified `property`
+
+# Output
+
+This function return a new `Physics` object
+
+# Example
+
+To change a model to run a transient simulation e.g. after converging in steady state
+
+ modelTransient = change(model, :time, Transient())
+"""
 function change(model::Physics, property, value)
  @assert property ∈ fieldnames(Physics) throw(ArgumentError(
  """$value is not in Physics.