Commit 480324ec "docs/qdev-device-use: Clean up the sentences
related to -usbdevice" deleted the information on syntax that no
longer works.  Unfortunately, the resulting text suggests you can
configure USB block devices with -drive, and USB network devices with
-net, which is misleading.

Instead of rephrasing the now misleading text, I'm putting the
information back, and just make clear it's about old versions of QEMU.

Cc: Thomas Huth <thuth@redhat.com>
Cc: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20200806081147.3123652-4-armbru@redhat.com>