Add more background explanation.

* Add a brief explanation of why it's not REST-based.
* Add a brief explanation of why it's not binary.
* Add mention of WebSockets as a transport protocol.
* Clarify that servers do not need to assign IMAP-like ascending UIDs to
  messages; just unique string ids.
* Remove reference to ISO 8601 for date format – whilst the format used is
  defined in that spec, the spec is huge and defines many others as well.