Update s-split documentation

The emacs documentation for split-string, which s-split wraps, is not correct:
"If OMIT-NULLS is t, zero-length substrings are omitted from the list (so
that for the default value of SEPARATORS leading and trailing whitespace
are effectively trimmed).  If nil, all zero-length substrings are retained,
which correctly parses CSV format, for example."

`omit-nulls` need not be `t`, it can be anything non-null:
```
...
  (let* ((keep-nulls (not (if separators omit-nulls t)))
...
```

This is useful to know if you prefer to use human-readable args, like `:omit-nulls` instead of `t`.