An extension method for AsClauseSyntax API isn't documented

https://github.com/dotnet/roslyn/blob/91571a3bb038e05e7bf2ab87510273a1017faed0/src/Compilers/VisualBasic/Portable/Syntax/SyntaxExtensions.vb#L48-L63

It's already a public API in roslyn. But the extension methods section doesn't show it.

cc: @gewarren

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#### Document Details

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* ID: ca494096-7838-4485-5b98-a7517b82049c
* Version Independent ID: f29ede21-fcc9-5e91-7840-d4ba7ccd474c
* Content: [AsClauseSyntax Class (Microsoft.CodeAnalysis.VisualBasic.Syntax)](https://docs.microsoft.com/en-us/dotnet/api/microsoft.codeanalysis.visualbasic.syntax.asclausesyntax?view=roslyn-dotnet#extension-methods)
* Content Source: [dotnet/xml/Microsoft.CodeAnalysis.VisualBasic.Syntax/AsClauseSyntax.xml](https://github.com/dotnet/roslyn-api-docs/blob/live/dotnet/xml/Microsoft.CodeAnalysis.VisualBasic.Syntax/AsClauseSyntax.xml)
* Product: **dotnet-roslyn-api**
* Technology: **microsoft.codeanalysis**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**