(affixation-function): Allow only three-element list elements

(affixation-function): Allow only three-element list elements

Restrict the definition of the `affixation-function`.  The function
must return a list of three element lists.  Since the
`affixation-function` is part of the widely used `completing-read` API
a simplification is helpful for both authors of completion UIs and
authors of completion tables.

* doc/lispref/minibuf.texi: Update documentation.
* lisp/minibuffer.el: Update documentation.
* lisp/simple.el (read-extended-command--affixation):
Return three-element lists.

https://lists.gnu.org/archive/html/emacs-devel/2021-04/msg01193.html