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

[calcium]

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.