Hi Arnold, I admit to not checking, but if this isn't documented, it should be.
It is. Along with approximately two zillion other caveats about @macro usage :). Also, I would suggest that makeinfo be modified to enforce this restriction as well. Historically, makeinfo's @macro has always accepted lots of things that texinfo.tex's @macro doesn't. This was construed (not by me) to be a feature, and I believe some documents rely on it (i.e., use texi2dvi -E or have different definitions), so I don't much want to change it now. Best, Karl