gir: Documentation on instance structs is not passed to the gir generator

Documentation comments on instance structs e.g. struct _WpCore is not passed to gnome.generate_gir() in meson.

This means that its currently not possible to add Since tags to objects and have it end up in the gir.

Source code docs go through gen-api-gtkdoc.py, but elements only go into the output file if .is_documented() is true on the element.

.is_documented() returns true when the briefdescription is nonempty, but instance structs seem to only have a detaileddescription, so they are not considered documented.

Should we just extend the definition of is_documented() to also check if a detailed description is set?