Caching discrepancy between API docs and TrainerInfo.WantCaching property

There are discrepancies in the docs for trainers on whether caching is required or not.

For example: [MatrixFactorizationTrainer](https://github.com/dotnet/machinelearning/blob/fbd1b93065b451401b1e3276e5ac65b9f303f90b/src/Microsoft.ML.Recommender/MatrixFactorizationTrainer.cs)

In the documentation, it says caching is required:

| Is caching required? | Yes |

However, the `TrainerInfo.WantCaching` property is set to false.

```csharp
_info = new TrainerInfo(normalization: false, caching: false);
```

https://github.com/dotnet/machinelearning/blob/fbd1b93065b451401b1e3276e5ac65b9f303f90b/src/Microsoft.ML.Recommender/MatrixFactorizationTrainer.cs#L370

To-Do:

- [x] Get list of trainers these discrepancies occur.