ada: Reject non-primitive operations in Finalizable aspect

The implementation does not support them and allowing them would not bring
any significant benefit.

gcc/ada/ChangeLog:

	* doc/gnat_rm/gnat_language_extensions.rst
	(Generalized Finalization): Document the new restriction.
	* sem_ch13.adb (Resolve_Finalizable_Argument): Adjust wording of
	error message.
	(Resolve_Finalization_Procedure.Is_Finalizable_Primitive): Require
	the procedure to be a primitive operation.
	* gnat_rm.texi: Regenerate.