Comment at commit
https://git.gnome.org/browse/gxml/commit/?id=f3f46220914150426db11a893c9f163b060f5021
No makes me sense because if you change the returned value type from Element to Document it is and API break I think.
I know you point out that 0.3 is unstable you must release a new 0.3.4 with the change, point it includes an API break and wait for feedback from users before release an stable branch, fixing issues by releasing some 0.5 to 0.6 before stable (may be called 0.3.90 to .91 as release candidates).