bug#59256: Emacs-guix tab-completion returns gexp error

[calcium via Bug reports for GNU Guix <bug-guix@gnu.org>]

The problem can be tracked back from elisp procedure `guix-pcomplete-search-in-help` in guix-pcomplete.el to `guix-help-string` in guix-misc.el, to this problematic code :
```
(guix-eval-read "(help-string )")
```

The procedure help-string exists, as can be seen below :
```
(guix-eval-read
  "(begin
   (use-modules (ice-9 format))
   (format #f \"~s\"
    help-string))") ;; => "#<procedure help-string commands>"
```

but the object that `(help-string)` returns seems incompatible with guile's own repl/shell ....

The solution might be to just change the `help-string` procedure to return another kind of object, or to fix guile's repl/shell for that kind of object.