Add short docstrings

[tlienart]

At the very least should list and explain fields.

• Clustering
  • KMeans
  • KMedoids
• GaussianProcesses
  • GPClassifier
• GLM
  • LinearRegressor
  • LinearBinaryClassifier
  • LinearCountRegressor
• MultivariateStats
  • RidgeRegressor
  • PCA
  • KernelPCA
  • ICA
• NaiveBayes
  • GaussianNBClassifier
  • MultinomialNBClassifier
• NearestNeighbors
  • KNNRegressor
  • KNNClassifier

For Sklearn, probably best is to just link to the documentation website.

Also use the new MLJBase.docstring like

MLJBase.docstring(::Type{<:KNNRegressor}) = "K-Nearest Neighbor Regressor, based on [NearestNeighbors](https::.....).\nDo `@load KNNRegressor` and `?KNNRegressor` for documentation."

[OkonSamuel]

@tlienart shouldn't this be edited since you've add docstrings for MultivariateStats models

[ablaom]

Rendered out of date by JuliaAI/MLJ.jl#913