add docs about constructor trait

Author: ablaom

docs/src/model_wrappers.md

@@ -27,3 +27,21 @@ EnsembleModel(model=tree, n=100)
 
 This is the only case in MLJ where positional arguments in a model constructor are
 allowed.
+
+## Handling generic constructors
+
+Model wrappers frequently have a public facing constructor with a name different from that
+of the model type constructed. For example, `TunedModel(model, ...)` is a constructor that
+will construct either an instance of `DeterministicTunedModel` or
+`ProbabilisticTunedModel`, depending on the type of `model`. In this case it is necessary
+to overload the `constructor` trait, which in that case looks like this:
+
+```julia
+MLJModelInterface.constructor(::Type{<:Union{
+    DeterministicTunedModel,
+	ProbabilisticTunedModel,
+	}}) = TunedModel
+```
+
+This allows the MLJ Model Registry to correctly associate model metadata to the
+constructor, rather than the (private) types.