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

[Jean-Christophe Helary]

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

I've checked subr.el and the only functions that specifically document an 
argument as optional in the doc string are:
alist-get
suppress-keymap
substitute-key-definition
add-hook
remove-hook
add-to-list (only 1 out of 2)
add-to-ordered-list
add-to-history
add-minor-mode
locate-library (2/3)
process-kill-without-query
read-passwd
read-char-choice
sit-for
momentary-string-display
sha1 (2/3)
match-substitute-replacement
subst-char-in-string
replace-regexp-in-string
define-mail-user-agent
set-transient-map
make-progress-reporter (3/5)

out of 52 defuns (and 2 defsubst) that use &optional.

Should we modify all the remaining docstrings so that they explicitly specify 
when an argument is optional?

Jean-Christophe