Document the notebook kernel websocket message spec

The messages coming on a websocket connection to the kernel bridge are slightly different than the plain kernel message spec. We should document these differences for notebook server clients.

For example:

1. Messages have a `channel` top-level field to indicate what channel the messages are coming from.
2. The notebook itself sends some messages, like kernel restarting messages.
3. The websocket wire format should be documented.

I'm sure there are more differences.

CC @minrk

Start by comparing the websocket kernel bridge messages with the plain kernel message specification. Document the top-level channel field, notebook-originated messages such as kernel restarting messages, and the websocket wire format; done means the differences relevant to notebook server clients are covered.

Written by the indexing model from the issue text.