qapi: machine.json: change docs regarding CPU topology

qapi: machine.json: change docs regarding CPU topology

Clarify roles of different architectures.
Also change things a bit in anticipation of additional members being
added.

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Nina Schoetterl-Glausch <nsg@linux.ibm.com>
Message-ID: <20231016183925.2384704-2-nsg@linux.ibm.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
[thuth: Updated some comments according to suggestions from Markus]
Signed-off-by: Thomas Huth <thuth@redhat.com>