Fix: remove unused create_viewer_session option

Since it is unused for now, don't bloat the ABI with that.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Doc: live protocol clarifications

Updates about the get_new_stream, the attach procedure, the create
viewer_session and the multiple attach.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>