Clarify connection pooling model (with history) in .NET docs

There is quite a bit of confusion regarding MUX and connection pooling in the .NET SDK, this ticket is to clarify the current behavior and discuss the tradeoffs/benefits in the documentation so that it's documented and available to all user.

A couple possible points to discuss:

• History of blocking and MUX connections
• What the current defaults are
• Why the changes were made (covered in first point likely)
• How to fallback to the older model
• Danger zone: configuring MUX with the wrong connection pool/IO strategy and vice-versa - there is a factory method that's overridable and you can do bad things here.

Should be added here: https://developer.couchbase.com/documentation/server/5.0/sdk/dotnet/managing-connections.html

and here https://developer.couchbase.com/documentation/server/5.0/sdk/dotnet/client-settings.html