(No Status) → Declined
There is an informative RELAX NG schema that describes the current xml file format, which you can use to learn about the xml file format.
It is however impossible to code various accounting constraints in such an xml schema (eg to enforce each transaction has a zero balance). For this reason it is formally strongly discouraged to use any external tool to modify the xml data, unless it does so via the GnuCash api.
So there is no incentive for the GnuCash developers to spend more effort on a more elaborate xml schema.