replace-match with optional fields

[Dave Love]

The doc string for replace-match is unclear, and the Lisp manual
doesn't mention this behaviour, but it think it's wrong:

The doc says

  `\N' means substitute what matched the Nth `\(...\)'.
       If Nth parens didn't match, substitute nothing.

I'd hope `nothing' means `substitute an empty string' rather than
`don't substitute'; I'd guess that was the intention.  It's not what
happens, though:

(let ((s "foo"))
  (string-match "\\(foo\\).*\\(bar\\)?" s)
  (replace-match "1: \\1, 2: \\2" t nil s))
  => "1: foo, 2: \\2"

This behaviour doesn't seem useful, in contrast to substituting an
empty string.