Fix up GObject interface documentation

Fix up GObject interface documentation

 * Document how to override interfaces already implemented
   in a base class, and also call those base class implementations
   from a derived reimplementation.
 * Don't recomend people use base_init() style functions to
   initialize interface signals and properties, use default_init()
   aka class_init() instead (as G_DEFINE_INTERFACE() uses).
 * The above solves the interface init called multiple times
   problem, so remove some needless naysaying about that.
 * Document default_init() in the interface initialization discussion
 * Linkify more stuff.
 * Remove some crud and typos

https://bugzilla.gnome.org/show_bug.cgi?id=675504