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

[Eli Zaretskii]

> From: Jean-Christophe Helary <jean.christophe.helary@gmail.com>
> Date: Sat, 3 Jun 2017 22:50:55 +0900
> Cc: stephen.berman@gmx.net,
>  26925@debbugs.gnu.org
> 
> >> -If the optional argument @var{trim}
> >> →
> >> +If @var{trim}
> >> 
> >> just to adapt the text to the description of the other arguments.
> > 
> > Thanks, but I see no problem with the current text.  We use both
> > styles intermittently.
> 
> It's up to you. I thought it was weird that all the other optional arguments 
> were not labeled as "optional" and only this one.

That's not what I see: grepping the ELisp manual for "optional
argument" comes up with more than 200 hits.  It's quite normal to use
that.