gobject: Document that classes/objects/interfaces are zero-filled

On initialisation, GObject guarantees to zero-fill
class/object/interface structures. Document this so people don’t spend
forever writing:
    my_object->priv->some_member = NULL;
    my_object->priv->some_other_member = NULL;

https://bugzilla.gnome.org/show_bug.cgi?id=729167

gobject/gtype.h

@@ -882,10 +882,14 @@ typedef void   (*GClassFinalizeFunc)         (gpointer         g_class,
  * A callback function used by the type system to initialize a new
  * instance of a type. This function initializes all instance members and
  * allocates any resources required by it.
+ *
  * Initialization of a derived instance involves calling all its parent
  * types instance initializers, so the class member of the instance
  * is altered during its initialization to always point to the class that
  * belongs to the type the current initializer was introduced for.
+ *
+ * The extended members of @instance are guaranteed to have been filled with
+ * zeros before this function is called.
  */
 typedef void   (*GInstanceInitFunc)          (GTypeInstance   *instance,
 					      gpointer         g_class);
@@ -897,6 +901,9 @@ typedef void   (*GInstanceInitFunc)          (GTypeInstance   *instance,
  * A callback function used by the type system to initialize a new
  * interface.  This function should initialize all internal data and
  * allocate any resources required by the interface.
+ *
+ * The members of @iface_data are guaranteed to have been filled with
+ * zeros before this function is called.
  */
 typedef void   (*GInterfaceInitFunc)         (gpointer         g_iface,
 					      gpointer         iface_data);