qemu-doc: Belatedly document QMP command deprecation

qemu-doc: Belatedly document QMP command deprecation

Commit 24fb413300 "qmp: Introduce blockdev-change-medium" (v2.5.0)
deprecated change.

Commit 2ff3025797 "migrate: move max-bandwidth and downtime-limit to
migrate_set_parameter" (v2.8.0) deprecated migrate_set_downtime and
migrate_set_speed.

These wre missed in commit eb22aeca65 "docs: document deprecation
policy & deprecated features in appendix" (v2.10.0).

Commit 73af8dd8d7 "migration: Make xbzrle_cache_size a migration
parameter" (v2.11.0) deprecated migrate-set-cache-size and
query-migrate-cache-size, but neglected to update qemu-doc.

Make up for that.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191024110237.30963-6-armbru@redhat.com>