Revisit System.Net.Security.NegotiateStream async examples

There are several issues with `System.Net.Security.NegotiateStream` async examples:

There are inconsistencies between C# and C++, VB type of API.

While the C# uses XxxAsync() calls the C++ and VB uses BeginXxx(), EndXxx() methods.

This leads to a situation that:
1) for BeginXxx() we have wrong C# example: https://docs.microsoft.com/en-us/dotnet/api/system.net.security.negotiatestream.beginread?view=netcore-3.1
2) for XxxAsync() methods we don't have linked examples although it is available

There are also references to missing C# sections while the reference is valid for C++, VB which leads to empty code block in C# view: https://docs.microsoft.com/en-us/dotnet/api/system.net.security.negotiatestream.endwrite?view=netcore-3.1

My recommendation is to maintain only XxxAsync methods for all languages as BeginXxx() and EndXxx() are a legacy way of asynchronous programming.