arrow

MINOR: [Docs][C#] Update C# documentation

Updating the C# README and feature status matrix to match the current implementation.

Also resolving the question in https://github.com/apache/arrow/issues/11367.

Closes #11378 from eerhardt/UpdateDocs

Authored-by: Eric Erhardt <eric.erhardt@microsoft.com>
Signed-off-by: Eric Erhardt <eric.erhardt@microsoft.com>