refactor: document check keyword flags, mark as public

Author: AayushSabharwal

docs/src/API/System.md

@@ -186,3 +186,17 @@ ModelingToolkit.is_bound
 ```@docs
 debug_system
 ```
+
+## Input validation
+
+The following values can be passed to the `check` keyword of `System` to toggle validation
+of input. Flags can be combined with bitwise `|` and `&`.
+
+```@docs
+ModelingToolkit.CheckAll
+ModelingToolkit.CheckNone
+ModelingToolkit.CheckComponents
+ModelingToolkit.CheckUnits
+```
+
+These can also be used by custom `AbstractSystem` subtypes.

src/ModelingToolkit.jl

@@ -336,7 +336,7 @@ export DynamicOptSolution
 @public is_diff_equation, Equality, linearize_symbolic, reorder_unknowns
 @public similarity_transform, inputs, outputs, bound_inputs, unbound_inputs, bound_outputs
 @public unbound_outputs, is_bound
-@public AbstractSystem
+@public AbstractSystem, CheckAll, CheckNone, CheckComponents, CheckUnits
 
 for prop in [SYS_PROPS; [:continuous_events, :discrete_events]]
     getter = Symbol(:get_, prop)

src/utils.jl

@@ -88,9 +88,31 @@ function readable_code(expr)
 end
 
 # System validation enums
+"""
+    const CheckNone
+
+Value that can be provided to the `check` keyword of `System` to disable checking of input.
+"""
 const CheckNone = 0
+"""
+    const CheckAll
+
+Value that can be provided to the `check` keyword of `System` to enable all input
+validation.
+"""
 const CheckAll = 1 << 0
+"""
+    const CheckComponents
+
+Value that can be provided to the `check` keyword of `System` to only enable checking of
+basic components of the system, such as equations, variables, etc.
+"""
 const CheckComponents = 1 << 1
+"""
+    const CheckUnits
+
+Value that can be provided to the `check` keyword of `System` to enable checking of units.
+"""
 const CheckUnits = 1 << 2
 
 function check_independent_variables(ivs)