document the fully_determined kwarg to structural_simplify

Author: MasonProtter

src/systems/systems.jl

@@ -8,16 +8,19 @@ $(SIGNATURES)
 
 Structurally simplify algebraic equations in a system and compute the
 topological sort of the observed equations. When `simplify=true`, the `simplify`
-function will be applied during the tearing process. It also takes kwargs
-`allow_symbolic=false` and `allow_parameter=true` which limits the coefficient
-types during tearing.
+function will be applied during the tearing process. The kwargs
+`allow_symbolic=false` and `allow_parameter=true` limit the coefficient
+types during tearing. The kwarg `fully_determined=true` controls whether or not
+an error will be thrown if the number of equations don't match the number of inputs,
+outputs, and equations.
 
 The optional argument `io` may take a tuple `(inputs, outputs)`.
 This will convert all `inputs` to parameters and allow them to be unconnected, i.e.,
 simplification will allow models where `n_unknowns = n_equations - n_inputs`.
 """
 function structural_simplify(
         sys::AbstractSystem, io = nothing; simplify = false, split = true,
+        allow_symbolic = false, allow_parameter = false, fully_determined = true,
         kwargs...)
     newsys′ = __structural_simplify(sys, io; simplify, kwargs...)
     if newsys′ isa Tuple