Fix some wrong documentation headers from the first name given to this interface.
https://bugzilla.gnome.org/show_bug.cgi?id=646744
The GstTagXmpWriter interface is to be implemented on elements that provide xmp serialization. It allows users to select which xmp schemas should be used on serialization. API: GstTagXmpWriter https://bugzilla.gnome.org/show_bug.cgi?id=645167