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

[Jean-Christophe Helary]

> 2017/06/05 0:46、Eli Zaretskii <eliz@gnu.org>のメール:
>> If you don't want to see that string change, just say so and I'll drop it.
> 
> I thought I did, in the very first response to your change suggestion.
> 
> But if that makes you so much unhappy, please go ahead and make the
> change.

It is not about me being happy or not, but about me learning what's correct and 
what is not. If you tell me that it is 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.

>> No, because in the function signature they are defined as optional.
> 
> And yet we usually do say explicitly in doc strings that optional
> arguments are optional.

Does that mean that you suggest we explicitly declare them optional in 
split-string ? Maybe that's something I can do.

Jean-Christophe