Handler chain plumbing documentation

[pweil-]

Pulling out this comment as a possible doc addition

A drawing showing the plumbing into the handler chain: https://docs.google.com/drawings/d/1KCqpg5XJPa7QTJPBKY2RO2Na6HGVspJtVepyqjBsu5o/edit

Originally posted by @sttts in https://github.com/kcp-dev/kcp/issues/333#issuecomment-1198015104