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

[eduardstan]

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

export ModalDecisionTree, ModalRandomForest, ModalAdaBoost

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

    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.