qapi/machine: Clean up documentation around CpuInstanceProperties

qapi/machine: Clean up documentation around CpuInstanceProperties

CpuInstanceProperties' doc comment describes its members as properties
to be passed to device_add when hot-plugging a CPU.

This was in fact the initial use of this type, with
query-hotpluggable-cpus: letting management applications find out what
properties need to be passed with device_add to hot-plug a CPU.

We've since added other uses: set-numa-node (commit 419fcdec3c1 and
f3be67812c2), and query-cpus-fast (commit ce74ee3dea6).  These are not
about device-add.

query-hotpluggable-cpus uses CpuInstanceProperties within
HotpluggableCPU.  Lift the documentation related to device-add from
CpuInstanceProperties to HotpluggableCPU.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240711112228.2140606-3-armbru@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>