docs: Update relayd architecture

docs: Update relayd architecture

Observed issue
==============

The sessiond sessions do not map one-to-one with relay
sessions. Rather, there can be one relay session associated with
each of the active consumers, e.g. ustconsumerd64, ustconsumerd32,
and kconsumerd of each lttng-sessiond sessions.

Solution
========

The phrasing of the the relay session has been updated to
"per-consumer". An additional mention is added to say that
attaching a viewer session to multiple lttng-sessiond
sessions is not supported.

Change-Id: I1df18c4e97c0ee9ec4ee17b3bf35c6e74c90774f
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>