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

[Eli Zaretskii]

> From: Jean-Christophe Helary <jean.christophe.helary@gmail.com>
> Date: Mon, 5 Jun 2017 00:14:14 +0900
> Cc: Michael Albinus <michael.albinus@gmx.de>,
>  26925@debbugs.gnu.org,
>  stephen.berman@gmx.net
> 
> 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.

> >> And btw, the docstring of `split-string' does not speak about TRIM as
> >> "optional argument" either.
> > 
> > Which is a mistake, IMO, because the doc string uses neither of the
> > above 2 patterns, and so makes it sound like these arguments are not
> > optional at all.
> 
> 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.