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