Additional documentation for the ML.Net types and components should live in xml docs

[sfilipi]

Through PR #455 we introduced XML strings in the codebase, that flow to the generated CSharpApi, to better document the trainers of the Microsoft.ML.Trainers namespace.

The final destination for those XML strings should be xml files, in the 'docs' folder, parallel to the src folder. Those documents should be referenced in the CSharp api, and through the code by xml tags.