Re: Simplification of `affixation-function`

[Juri Linkov]

> You can also note that the third (suffix) element is always an empty string
> in both of these use cases. So affixation-function was really added to be
> able to add a prefix.

affixation-function was an improvement over annotation-function,
but nonetheless it has limitations too.  What would be a better thing is
like Daniel proposed a new meta `group-function`, I'd imagine a similar
meta `format-function` that could receive a candidate and return
a string to insert to the completions buffer.

Then the caller e.g. help--symbol-completion-table could define whether
to append "u", "a" and "c" in parens by using on a candidate something like
(format "%s (%s)" cand (cond ((fboundp (intern cand)) "f")) ...),
or prepend a dimmed letter as a prefix, or to use an icon.

The same `format-function` could be used to remove the group prefix
when `group-function` is in use, instead of providing an additional
argument `transform` for `group-function`.