gerror: Add an extra heading to the GError documentation
gerror: Add an extra heading to the GError documentation

This means that the top of the documentation can link forward to this
important section, and random people on the internet can link directly
to it on developer.gnome.org.

https://bugzilla.gnome.org/show_bug.cgi?id=743011

johne53/MB3Glib,ieei/glib,johne53/MB3Glib,johne53/MB3Glib,cention-sany/glib,cention-sany/glib,johne53/MB3Glib,tchakabam/glib,ieei/glib,MathieuDuponchelle/glib,Distrotech/glib,endlessm/glib,tchakabam/glib,MathieuDuponchelle/glib,Distrotech/glib,cention-sany/glib,ieei/glib,MathieuDuponchelle/glib,johne53/MB3Glib,tchakabam/glib,mzabaluev/glib,cention-sany/glib,tchakabam/glib,mzabaluev/glib,mzabaluev/glib,johne53/MB3Glib,mzabaluev/glib,endlessm/glib,Distrotech/glib,Distrotech/glib,tchakabam/glib,cention-sany/glib,endlessm/glib,endlessm/glib,mzabaluev/glib,ieei/glib,ieei/glib,MathieuDuponchelle/glib,endlessm/glib,Distrotech/glib,MathieuDuponchelle/glib