bug#23550: 25.0.93; cl.texi (for var on list by func): Fix documentation

[Tino Calancha <tino.calancha@gmail.com>]

npostavs@users.sourceforge.net writes:

>>
>> emacs -Q:
>>
>> (require 'cl-lib)
>> (setq preys '("grass" "lion" "rabbit"))
>>
>> (defun next-prey (x)
>>   "Return the name of the prey of animal X."
>>   (cl-loop while (not (atom x))
>>            do (setq x (car x)))
>>   (cond ((member x preys) ; elisp
>>   ;; (cond ((member x preys :test #'string=) ; CL
>>          (cond ((string= x "lion") "rabbit")
>>                ((string= x "rabbit") "grass")
>>                (t nil)))
>>         (t
>>          nil)))
>> (defun next-prey-list (x)
>>   "Return a list with the name of the prey of animal X."
>>   (let ((res (next-prey x)))
>>     (if res
>>         (list res)
>>       nil)))
>
> I guess the idea behind that statement is that the rest of the list can
> be implied by the stepping function, e.g.
>
>     (cl-loop for y on (list "lion") by #'next-prey-list collect (car y))
>       ;=> ("lion" "rabbit" "grass")
>
> Works with the "in" clause too:
>
>     (cl-loop for y in (list "lion") by #'next-prey-list collect y)
>       ;=> ("lion" "rabbit" "grass")
>
> I guess it could be useful in combination with streams?  Not sure if
> it's worth having this in the manual.
In the way is written makes me expect that:
(cl-loop for y on "lion" by #'next-prey-list collect (car y))

will be evaluated to: ("lion" "rabbit" "grass").

Becase i read in the example first-animal, instead of (list first-animal), 
and they also write:
'With @code{by}, there is no real reason that the @code{on} expression
must be a list.'

It might be useful rewrite that part to make it more clear, but i don't
have a clear proposal.