Missing description and example

It'd be helpful to have a description of this method as well as an example. Thanks!

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#### Document Details

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* ID: c003a66d-1e1a-c2d1-459a-910c21205151
* Version Independent ID: cf53de3d-5293-79c3-4e23-c00860848139
* Content: [StringBuilder.AppendJoin Method (System.Text)](https://docs.microsoft.com/en-us/dotnet/api/system.text.stringbuilder.appendjoin?view=netcore-2.2#definition)
* Content Source: [xml/System.Text/StringBuilder.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.Text/StringBuilder.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**