Provide Information about How To Obtain Entry-Block Reference

This page does not state that the first block of the Blocks property is the entry block and provides no further hint on how to obtain the entry block. I gathered this information from the BasicBlockKind enum documentation, but it took some time and research.

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* ID: ddc992a9-1927-9fac-e209-c07a4f0f26cd
* Version Independent ID: 41aef72e-ac29-78d2-b957-b56d4e326edb
* Content: [ControlFlowGraph Class (Microsoft.CodeAnalysis.FlowAnalysis)](https://learn.microsoft.com/en-us/dotnet/api/microsoft.codeanalysis.flowanalysis.controlflowgraph?view=roslyn-dotnet-4.3.0)
* Content Source: [dotnet/xml/Microsoft.CodeAnalysis.FlowAnalysis/ControlFlowGraph.xml](https://github.com/dotnet/roslyn-api-docs/blob/live/dotnet/xml/Microsoft.CodeAnalysis.FlowAnalysis/ControlFlowGraph.xml)
* Product: **dotnet-roslyn-api**
* Technology: **microsoft.codeanalysis**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**