audio: add -audiodev pa,in|out.latency= to documentation

The "latency" parameter wasn't covered by the documentation.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-id: 20191004125641.24383-1-stefanha@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>