`ModalAdaBoost` has no docstring while its two sibling models do

`src/interfaces/MLJ.jl` exports three MLJ models:

```julia
export ModalDecisionTree, ModalRandomForest, ModalAdaBoost
```

`src/interfaces/MLJ/docstrings.jl` generates documentation by branching on the model type, and the
branches present are:

```julia
if T <: ModalDecisionTree
...
elseif T <: ModalRandomForest
```

`ModalAdaBoost` has no branch, so the third exported model appears to reach users undocumented while
the other two are covered.

That matters more than a typical missing docstring, because MLJ users routinely discover a model's
hyperparameters through its docstring rather than by reading source - so the model is harder to use
than its siblings for a reason unrelated to the model itself.

### What would close it

A `ModalAdaBoost` branch in the same generator, following the shape of the two that exist.

Noticed during an outside survey of the ecosystem; if the omission is deliberate because the model is
provisional, saying so would answer it just as well.