Finalizer documentation, minor improvements

Finalizer documentation, minor improvements

* doc/lispref/objects.texi (Finalizer Type): New section
(Type Predicates): Mention finalizers in `type-of' documentation.
* doc/lispref/elisp.texi (Top): Link to finalizer type.

* src/data.c (Ftype_of): Make `type-of' work with finalizers.
(syms_of_data): Register Qfinalizer.

* src/print.c (print_object): Print whether a finalizer has
been called.

* test/automated/finalizer-tests.el (finalizer-object-type): Test that
`type-of' works correctly for finalizers.