In glib, GTypeCValue struct is missing from the doc.

I believe this is because gtkdoc-scan first see in gtype.h:
typedef union  _GTypeCValue             GTypeCValue;

It adds it to $forward_decls, then at the end of the file it didn't see the declaration so it writes in gobject-decl.txt:

<UNION>
<NAME>GTypeCValue</NAME>
</UNION>

Later it parses gvaluecollector.h and see:
union _GTypeCValue
{
  gint     v_int;
  glong    v_long;
  gint64   v_int64;
  gdouble  v_double;
  gpointer v_pointer;
};

It writes in gobject-decl.txt the whole thing.

Later it removes dups from gobject-decl.txt and the 2nd, more complete, is lost.