Documentation of IncrementalGeneratorInitializationContext is missing

Can each of the exposed methods get a few lines of documentation explaining each and when to use them? It isn't clear and there is nowhere else explaining this.

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* ID: 96619643-e973-424b-61d8-046798054662
* Version Independent ID: 96619643-e973-424b-61d8-046798054662
* Content: [IncrementalGeneratorInitializationContext Struct (Microsoft.CodeAnalysis)](https://docs.microsoft.com/en-us/dotnet/api/microsoft.codeanalysis.incrementalgeneratorinitializationcontext?view=roslyn-dotnet-4.0.1)
* Content Source: [dotnet/xml/Microsoft.CodeAnalysis/IncrementalGeneratorInitializationContext.xml](https://github.com/dotnet/roslyn-api-docs/blob/live/dotnet/xml/Microsoft.CodeAnalysis/IncrementalGeneratorInitializationContext.xml)
* Product: **dotnet-roslyn-api**
* Technology: **microsoft.codeanalysis**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**