Hello George,
The a:documentation is described in the RELAX NG DTD Compatibility specification: http://www.oasis-open.org/committees/relax-ng/compatibility.html#IDAC1YR
It does not allow any elements inside, only text. Note however that in a Relax NG grammar any element not in the Relax NG namespace is considered annotation.
I have to admit that I don't understand your answer fully. The link above tells me, that in <a:documentation> no element is allowed. Right? The second part of your answer is what I don't understand. I can, for example, use <br /> in <a:documentation>. That gives me a linebreak in Oxygen. I'd guess that this is non-official?!? I only need it for Oxygen use, so I could live with that. Any more elements Oxygen understands in <a:documentation>? Or is this a "wrong" question? Patrick