Isolate documentation for incidental public types

[samalone]

There are several types in the package that are public not because they are intended to be used directly, but because they are required to be public as part of the DistributedActorSystem protocol requirements. These include:

• NIOInvocationEncoder
• NIOInvocationDecoder
• WebSocketActorSystemResultHandler

These types should be grouped into a section at the bottom of the top-level documentation so they don't distract from the primary APIs.