qapi: document parameters of query-cpu-model-* QAPI commands

qapi: document parameters of query-cpu-model-* QAPI commands

Let's document the parameters of these commands, so we can remove them
from the "documentation-exceptions" list.

While at it, extend the "Returns:" documentation as well, fixing a wrong
use of CpuModelBaselineInfo vs. CpuModelCompareInfo for
query-cpu-model-comparison.

Cc: Markus Armbruster <armbru@redhat.com>
Cc: Eric Blake <eblake@redhat.com>
Cc: Eduardo Habkost <eduardo@habkost.net>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: "Philippe Mathieu-Daudé" <philmd@linaro.org>
Cc: Yanan Wang <wangyanan55@huawei.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-ID: <20240325150141.342720-1-david@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
[Punctuation tweaked]
Signed-off-by: Markus Armbruster <armbru@redhat.com>