'vtable-update-object' can now be called with one argument

'vtable-update-object' can now be called with one argument

It's often necessary to update the representation of a single
object in a table (e.g a struct, whose identity does not change
when its slots'
values are changed).  To do so, now the function may be called
like this:

  (vtable-update-object table object)

Instead of like this:

  (vtable-update-object table object object)

This also documents the behavior of the just-discovered limitation filed
as bug#69837.
* lisp/emacs-lisp/vtable.el (vtable-update-object): Make 'old-object'
argument optional.  (Bug#69666)

* doc/misc/vtable.texi (Interface Functions): Update documentation.

* etc/NEWS: Add news entry.