bug#26925: Improve /doc/lispref/strings.texi (split-string) documentation

[Jean-Christophe Helary]

> 2017/06/05 11:46、Eli Zaretskii <eliz@gnu.org>のメール:
> 
>> not correct to worry about various styles being used seemingly at random, 
>> then I learn from that. If you tell me that it is not necessary, which you 
>> did, then I can argue that I feel better if the few lines I was involved 
>> with are tidied up.
> 
> I'm not sure I understand the last sentence, but if I do, then no, you
> shouldn't worry about random changes of style, as long as the text is
> accurate and is correct English.  This is, after all, descriptive
> text in plain English, not a formal document.

Ok, I understand, it was just that I felt I could make my original modification 
look nicer. I drop it. Thank you.

>> Does that mean that you suggest we explicitly declare them optional in 
>> split-string ?
> 
> Yes, I think so.

Ok, I'll do it.

Jean-Christophe