Revisit System.Net.Security.NegotiateStream async examples

[aik-jahoda]

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.

[carlossanlop]

Seems to be fixed with #4335 so I'm closing this. Feel free to reopen if there are still items that need to be addressed.

[aik-jahoda]

There are still wrong examples in the documentation.

BeginRead uses ReadAsync example:
https://docs.microsoft.com/en-us/dotnet/api/system.net.security.negotiatestream.beginread?view=netcore-3.1