feat: document and expose MTKParameters as public API

Author: AayushSabharwal

docs/src/internals.md

@@ -35,3 +35,12 @@ The call chain typically looks like this, with the function names in the case of
  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`.
+
+## Creating an `MTKParameters` object
+
+It may be useful to create a parameter object without creating the problem. For this
+purpose, the `MTKParameters` constructor is exposed as public API.
+
+```@docs
+MTKParameters
+```

src/ModelingToolkit.jl

@@ -275,4 +275,6 @@ export debug_system
 export Sample, Hold, Shift, ShiftIndex, sampletime, SampleTime
 export Clock, SolverStepClock, TimeDomain
 
+export MTKParameters
+
 end # module

src/systems/parameter_buffer.jl

@@ -10,9 +10,23 @@ struct MTKParameters{T, D, C, N}
     nonnumeric::N
 end
 
+"""
+    function MTKParameters(sys::AbstractSystem, p, u0 = Dict(); t0 = nothing)
+
+Create an `MTKParameters` object for the system `sys`. `p` (`u0`) are symbolic maps from
+parameters (unknowns) to their values. The values can also be symbolic expressions, which
+are evaluated given the values of other parameters/unknowns. `u0` is only required if
+the values of parameters depend on the unknowns. `t0` is the initial time, for time-
+dependent systems. It is only required if the symbolic expressions also use the independent
+variable of the system.
+
+This requires that `complete` has been called on the system (usually via
+`structural_simplify` or `@mtkbuild`) and the keyword `split = true` was passed (which is
+the default behavior).
+"""
 function MTKParameters(
         sys::AbstractSystem, p, u0 = Dict(); tofloat = false, use_union = false,
-        t0 = nothing, eval_expression = false, eval_module = @__MODULE__)
+        t0 = nothing)
     ic = if has_index_cache(sys) && get_index_cache(sys) !== nothing
         get_index_cache(sys)
     else