Revise NETWORK_LIVENESS spec

Minor revisions for the new NETWORK_LIVENESS event...

  * Include a note about the version when this is being added.

  * No need to say new fields are allowed. This is said near the start of
    section 4.1...

    "Clients MUST tolerate events with arguments and keywords they do not
    recognize"

  * Rejigger the spec's style a tad to be more like other events.