So far gtk-doc included a heuristic that accepted the last paragraph of
a body doc-blob as return documentation if it started with "Return(s)".
As this can be ambiguous with a desired "Returns: " block, we added a
warning for the cases the heuristic kicked in many years ago. I just
pushed a change to drop the heuristics.
If you now build your docs using gtk-doc HEAD and get entries in
$(DOC_MODULE)-undocumented.txt, check the symbols and convert them into
proper Returns: sections.