Improve the documentation on the trainer classes.

[sfilipi]

The documentation describing the trainers on docs.microsoft.com needs to be expanded.

[pkulikov]

Is it going to be a large overlap of the remarks in API with the topic that is under construction in dotnet/docs#5698?

[shauheen]

Closing this as the main PRs are merged and issue #477 is open to make further enhancements

[sfilipi]

@pkulikov the two efforts will converge. Thanks for pointing out that particular PR.