gtype: Improve documentation for G_TYPE_IS_CLASSED and interfaces

The documentation previously implicitly said in a few places that
interfaces are classed, but reading through the implementation of
`GType`, I don’t think they are. If they were, the registration of the
fundamental `G_TYPE_INTERFACE` in `gobject_init()` would specify
`G_TYPE_FLAG_CLASSED`. It only specifies `G_TYPE_FLAG_DERIVABLE`.

I think this makes sense, because you can’t subclass an interface.
Subclassing is a key property of being classed.

Tweak the `GType` tutorial to remove that implicit statement, and expand
the documentation for `G_TYPE_IS_CLASSED`.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Fixes: #252

gobject/gtype.h

@@ -304,6 +304,15 @@ G_BEGIN_DECLS
  * 
  * Checks if @type is a classed type.
  *
+ * A classed type has an associated #GTypeClass which can be derived to store
+ * class-wide virtual function pointers and data for all instances of the type.
+ * This allows for subclassing. All #GObjects are classed; none of the scalar
+ * fundamental types built into GLib are classed.
+ *
+ * Interfaces are not classed: while their #GTypeInterface struct could be
+ * considered similar to #GTypeClass, and classes can derive interfaces,
+ * #GTypeInterface doesn’t allow for subclassing.
+ *
  * Returns: %TRUE if @type is classed
  */
 #define G_TYPE_IS_CLASSED(type)                 (g_type_test_flags ((type), G_TYPE_FLAG_CLASSED))