System.IO.Pipelines is seriously underdocumented

Given that there's movement towards exposing a second implementation of IBufferWriter within the timeframe of .NET Core 3.0, we should probably provide documentation on Use-Case and differentiation with others specifically for PipeWriter.

Currently all framework spices redirect to .NET Platform Extensions, where no exposed type is really documented besides some sparse summaries.

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#### Document Details

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* ID: 7f22281d-14f2-973d-7f47-74d00c35e925
* Version Independent ID: 4bb0d319-3f73-2acb-4044-a6b85840e0e4
* Content: [PipeWriter Class (System.IO.Pipelines)](https://docs.microsoft.com/en-us/dotnet/api/system.io.pipelines.pipewriter?view=dotnet-plat-ext-2.1)
* Content Source: [xml/System.IO.Pipelines/PipeWriter.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.IO.Pipelines/PipeWriter.xml)
* Product: **dotnet-api**
* GitHub Login: @dotnet-bot
* Microsoft Alias: **dotnetcontent**