Updates man pages with the suboptions for the `-display`.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/128
Buglink: https://bugs.launchpad.net/qemu/+bug/1620660


Signed-off-by: Ahmed Abouzied <email@aabouzied.com>
Message-Id: <20210601174117.661-1-email@aabouzied.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>