Add note for calibrated trainers

Use information from issue dotnet/machinelearning#6092 to add a note about casting depending on whether a model is calibrated or not.

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#### Document Details

⚠ *Do not edit this section. It is required for docs.microsoft.com ➟ GitHub issue linking.*

* ID: 0366cc46-bc07-44fd-72d9-d37b40bd5eca
* Version Independent ID: 5c20d041-f946-d7c8-b0b5-7bee96e90282
* Content: [Re-train a model - ML.NET](https://docs.microsoft.com/en-us/dotnet/machine-learning/how-to-guides/retrain-model-ml-net)
* Content Source: [docs/machine-learning/how-to-guides/retrain-model-ml-net.md](https://github.com/dotnet/docs/blob/main/docs/machine-learning/how-to-guides/retrain-model-ml-net.md)
* Product: **dotnet-ml**
* GitHub Login: @luisquintanilla
* Microsoft Alias: **luquinta**