docs: fix up reference docs a bit

- require GTK-Doc 1.19
      - remove sgml mode
      - automatically generate gi.types (needs GTK-Doc 1.19)
      - fix https://bugzilla.gnome.org/show_bug.cgi?id=700025
[WIP] - rearange sections a bit
[WIP] - add gi-building, gi-programming sections
[WIP] - mark missing docs with TODO, which is only marginaly
        better than nothing but at least can be grepped :)

https://bugzilla.gnome.org/show_bug.cgi?id=571648

givfuncinfo.h

@@ -31,6 +31,12 @@
 
 G_BEGIN_DECLS
 
+/**
+ * GI_IS_VFUNC_INFO
+ * @info: an info structure
+ *
+ * Checks if @info is a #GIVfuncInfo.
+ */
 #define GI_IS_VFUNC_INFO(info) \
     (g_base_info_get_type((GIBaseInfo*)info) ==  GI_INFO_TYPE_VFUNC)