- Change screenco to screen + co since the former is not supported

properly with xsltproc and the latter is simpler
- Add a constraint to prevent using screenco, programlistingco and
  graphicco in the future
- Note that mediaobjectco is used in the documentation to properly
  compose a figure element but the callout bugs are already there
  in the source image and not placed there by the stylesheets

share/xml/freebsd.sch

@@ -8,4 +8,15 @@
       <assert test="contains(., '.')">File reference does not have an extension.</assert>
     </rule>
   </pattern>
+
+  <pattern name="Check callout validity">
+    <rule context="/">
+      <assert test="//screenco">Callouts with screenco are not supported;
+	use screen and co instead.</assert>
+      <assert test="//programlistingco">Callouts with programlistingco
+	are not supported; use programlisting and co instead.</assert>
+      <assert test="//graphicco">Callouts on
+	graphics are not supported.</assert>
+    </rule>
+  </pattern>
 </schema>