Documentation request: test data + model order

Opening issue to address questions on data splits and iterations posted on dotnet/machinelearning-modelbuilder repo.

Original Issue Description:

> Is your feature request related to a problem? Please describe.
> I am concerned about how to evaluate the performance of the models as the documentation does not clearly explain how the > test data is split. I also would like to estimate better how long to run iterations if I have already experienced with a related data set.
>
> Describe the solution you'd like
> I suggest adding to documentation:
>
> how is test data split. E.g. is it from the end, randomly or based on auto-selection of columns?
> how are the iterations generated. It would seem they might be in fixed order, or my data is fairly similar and leads to it. For
> example, if iteration 201 performed best, then when I re-train I could stop after iteration 201 next time, if I know this. If they are not in fixed order, then I know to let my next dataset run a bit longer.

For more details, see dotnet/machinelearning-modelbuilder#1530

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#### Document Details

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* ID: f35b206b-87eb-cca0-8854-d1fcd6d8732f
* Version Independent ID: 11c11e7b-5c02-ee1a-e17d-f5cb4cafbab8
* Content: [What is Model Builder and how does it work? - ML.NET](https://docs.microsoft.com/en-us/dotnet/machine-learning/automate-training-with-model-builder)
* Content Source: [docs/machine-learning/automate-training-with-model-builder.md](https://github.com/dotnet/docs/blob/main/docs/machine-learning/automate-training-with-model-builder.md)
* Product: **dotnet-ml**
* GitHub Login: @luisquintanilla
* Microsoft Alias: **luquinta**