Add short docstrings

[tlienart]

At the very least should list and explain fields.

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