doclets/gtkdoc: Print a warning if gtk-doc sections will be missing a long description
doclets/gtkdoc: Print a warning if gtk-doc sections will be missing a long description

If a Valadoc comment is used as the description for a gtk-doc section,
gtk-doc will emit a warning if only a short description is provided.
This pseudo-forwards that warning to the Valadoc output (from the
*-undocumented.txt file) so it doesn’t get lost.

frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,frida/vala