Fix alignment and documentation of vtable.el

Fix alignment and documentation of vtable.el

* lisp/emacs-lisp/vtable.el (vtable--insert-header-line): Ensure
proper alignment between the columns in header-line and in the
body of the table.  (Bug#73032)

* doc/misc/vtable.texi (Making A Table): Document the defaults of
the various keyword parameters.

(cherry picked from commit 04c44405bf604380c575fa2a4d9611af0f3bc0d9)