Stream vs Accessor

Please add some info about when to use `MemoryMappedViewAccessor` or ~`MemoryMappedViewAccessor`~ `MemoryMappedViewStream` to read/write data to the memory-mapped file.
I'm interested in performance differences the most.

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* ID: 4e773de7-e156-1dd3-b13e-2355104739b7
* Version Independent ID: 0b15055f-a2a5-2fc0-1d12-53d4d0ca86c3
* Content: [MemoryMappedFile Class (System.IO.MemoryMappedFiles)](https://docs.microsoft.com/en-us/dotnet/api/system.io.memorymappedfiles.memorymappedfile?view=netframework-4.7.2)
* Content Source: [xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml](https://github.com/dotnet/dotnet-api-docs/blob/master/xml/System.IO.MemoryMappedFiles/MemoryMappedFile.xml)
* Product: **dotnet-api**
* GitHub Login: @mairaw
* Microsoft Alias: **mairaw**