Small change in doc snarf output

[Neil Jerram]

I just noticed a small change in the doc snarfer output, in the way
that it denotes multiple optional args.

For example, the doc for `string-index' used to begin:

  - primitive: string-index str chr [frm [to]]

and now begins:

  - primitive: string-index str chr [frm to]

Personally, I prefer how it used to be.  (It appeals to my theoretical
side.)

So, was this change intentional, and what does everyone else prefer?

        Neil