cmdline-opts/socks*: Mention --preproxy in --socks* opts

- Document in --socks* opts they're still mutually exclusive of --proxy.

Partial revert of 423a93ce; I had misinterpreted the SOCKS proxy +
HTTP/HTTPS proxy combination.

- Document in --socks* opts that --preproxy can be used to specify a
  SOCKS proxy at the same time --proxy is used with an HTTP/HTTPS proxy.