Move SOCKSPort open proxy warning to a more sensible location

The SOCKSPort open proxy warning in the manual page is just below PreferIPv6.

It should probably be just under the first SOCKSPort paragraph. (Or, perhaps, right at the end of the flags.)

Here is what it says:

NOTE: Although this option allows you to specify an IP address
               other than localhost, you should do so only with extreme
               caution. The SOCKS protocol is unencrypted and (as we use it)
               unauthenticated, so exposing it in this way could leak your
               information to anybody watching your network, and allow anybody
               to use your computer as an open proxy.