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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