I don't know anything about texinfo or sphinx but can probably figure it out.
Looking at the sphinx documentation, I see that it can generate texinfo documents--is there a similar conversion from texinfo to sphinx? Is that what you want documented?
If there's not already a conversion from texinfo to sphinx, would it be worth the effort to make one?
Looking at the Sphinx main page I see that as long as documentation can be converted to xhtml, it can then be converted to restructured text.
seems to suggest that html can be converted to xhtml.