Merge pull request #3380 from vyudu/discrete_docs

add documentation page for `DiscreteSystem`

Author: ChrisRackauckas

docs/pages.jl

@@ -41,7 +41,8 @@ pages = [
         "systems/JumpSystem.md",
         "systems/NonlinearSystem.md",
         "systems/OptimizationSystem.md",
-        "systems/PDESystem.md"],
+        "systems/PDESystem.md",
+        "systems/DiscreteSystem.md"],
     "comparison.md",
     "internals.md"
 ]

docs/src/systems/DiscreteSystem.md

@@ -0,0 +1,28 @@
+# DiscreteSystem
+
+## System Constructors
+
+```@docs
+DiscreteSystem
+```
+
+## Composition and Accessor Functions  
+
+- `get_eqs(sys)` or `equations(sys)`: The equations that define the discrete system.  
+- `get_unknowns(sys)` or `unknowns(sys)`: The set of unknowns in the discrete system.  
+- `get_ps(sys)` or `parameters(sys)`: The parameters of the discrete system.  
+- `get_iv(sys)`: The independent variable of the discrete system
+- `discrete_events(sys)`: The set of discrete events in the discrete system.
+ 
+## Transformations
+
+```@docs; canonical=false
+structural_simplify
+```
+ 
+## Problem Constructors
+
+```@docs; canonical=false
+DiscreteProblem(sys::DiscreteSystem, u0map, tspan)
+DiscreteFunction(sys::DiscreteSystem, args...)
+```

src/systems/discrete_system/discrete_system.jl

@@ -323,6 +323,19 @@ end
 function SciMLBase.DiscreteFunction{false}(sys::DiscreteSystem, args...; kwargs...)
     DiscreteFunction{false, SciMLBase.FullSpecialize}(sys, args...; kwargs...)
 end
+
+"""
+```julia
+SciMLBase.DiscreteFunction{iip}(sys::DiscreteSystem,
+                            dvs = unknowns(sys),
+                            ps = parameters(sys);
+                            kwargs...) where {iip}
+```
+
+Create an `DiscreteFunction` from the [`DiscreteSystem`](@ref). The arguments `dvs` and `ps`
+are used to set the order of the dependent variable and parameter vectors,
+respectively.
+"""
 function SciMLBase.DiscreteFunction{iip, specialize}(
         sys::DiscreteSystem,
         dvs = unknowns(sys),