Add C# snippets to high level multiplayer API docs

[CarlCochet]

Add C# snippets to high level multiplayer docs

[AThousandShips]

Thank you for your contribution!

You should target master not 4.3, changes can then be cherry picked, see here.

Please open a new PR and a new branch based on the correct branch, if needed (check the latest version first).

[raulsntos]

Please note that these snippets have already been ported to C# in #9798. The PR has the cherry-pick:4.3 label which means it will be backported to the 4.3 documentation eventually, but this hasn't happened yet. Thanks for the contribution nonetheless, and feel free to improve the snippets if there's something in these changes that is missing in master.

[CarlCochet]

Thanks for the heads up! I completely forgot to switch on latest versions of docs.
I reviewed the added snippets and I don't think I have changes to bring, they all look great.