docs: add doc for generate_custom_function

Author: AayushSabharwal

docs/src/internals.md

@@ -32,6 +32,6 @@ The call chain typically looks like this, with the function names in the case of
 
  1. Problem constructor ([`ODEProblem`](@ref))
  2. Build an `DEFunction` ([`process_DEProblem`](@ref) -> [`ODEFunction`](@ref)
- 3. Write actual executable code ([`generate_function`](@ref))
+ 3. Write actual executable code ([`generate_function`](@ref) or [`generate_custom_function`](@ref))
 
 Apart from [`generate_function`](@ref), which generates the dynamics function, `ODEFunction` also builds functions for observed equations (`build_explicit_observed_function`) and Jacobians (`generate_jacobian`) etc. These are all stored in the `ODEFunction`.

src/systems/abstractsystem.jl

@@ -145,8 +145,26 @@ Generate a function to evaluate the system's equations.
 """
 function generate_function end
 
+"""
+```julia
+generate_custom_function(sys::AbstractSystem, exprs, dvs = unknowns(sys),
+                         ps = full_parameters(sys); kwargs...)
+```
+
+Generate a function to evaluate `exprs`. `exprs` is a symbolic expression or
+array of symbolic expression involving symbolic variables in `sys`. The symbolic variables
+may be subsetted using `dvs` and `ps`. All `kwargs` except `postprocess_fbody` and `states`
+are passed to the internal [`build_function`](@ref) call. The returned function can be called
+as `f(u, p, t)` or `f(du, u, p, t)` for time-dependent systems and `f(u, p)` or `f(du, u, p)`
+for time-independent systems. If `split=true` (the default) was passed to [`complete`](@ref),
+[`structural_simplify`](@ref) or [`@mtkbuild`](@ref), `p` is expected to be an `MTKParameters`
+object.
+"""
 function generate_custom_function(sys::AbstractSystem, exprs, dvs = unknowns(sys),
         ps = parameters(sys); wrap_code = nothing, kwargs...)
+    if !iscomplete(sys)
+        error("A completed system is required. Call `complete` or `structural_simplify` on the system.")
+    end
     p = reorder_parameters(sys, ps)
     isscalar = !(exprs isa AbstractArray)
     if wrap_code === nothing