scanner: Add fallback docs for virtual functions
Many virtual functions definitions are still documented using the gtk-doc struct field style, as opposed to a full docblock near the function pointer.
We should still favour the full docblock, as it can contain an appropriate description and annotations for arguments and return values; but if there is no block we can at least try to see if there's a field description to fall back to, to avoid fully undocumented symbols.
Fixes: #478 (closed)