<equation-number> not yet in the spec

I just noticed while building the spec that we have <equation-number> in the content models, but there is no topic for it in the language reference. Sending this note to the TC just to make sure we don't forget to add it.

Thanks -

Robert D Anderson

IBM Authoring Tools Development

Chief Architect, DITA Open Toolkit (http://dita-ot.sourceforge.net/)