exampleWeightColumnName parameter name and description may be confusing

The purpose or use of this parameter may be confusing for users as to what the intent of it is based on the name and description.

- It would be good to keep the name inline with the other column names (i.e. featureColumnName, labelColumnName).
- Adding what type is expected by the column in the description would help as well (The column data must be Single).
- Finally, it would help to describe the function of this parameter. Is this applying a weight to the individual observation, is it meant to set the initial weights of the features, or is it meant to set the importance of the features? To my knowledge this is to set the importance of the individual observation but clarification would help.

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* ID: ca42bd00-b107-5a38-06c2-5b33e5e361fc
* Version Independent ID: 9eceb504-86f7-3e71-7ddd-97e21254c026
* Content: [StandardTrainersCatalog.Sdca Method (Microsoft.ML)](https://docs.microsoft.com/en-us/dotnet/api/microsoft.ml.standardtrainerscatalog.sdca?view=ml-dotnet#feedback)
* Content Source: [dotnet/xml/Microsoft.ML/StandardTrainersCatalog.xml](https://github.com/dotnet/ml-api-docs/blob/live/dotnet/xml/Microsoft.ML/StandardTrainersCatalog.xml)
* Product: **dotnet-ml-api**
* GitHub Login: @sfilipi
* Microsoft Alias: **johalex**