Exits don't allow single-hop SOCKSPorts - update single onion man page
The single onion service manual page entries encourage users to use Tor2Web when opening a SOCKSPort on a tor in single onion service mode.
But this SOCKSPort will only work as a hidden service client, and not an exit client, because exits block single-hop circuits.
So we should fix the log message and the docs to avoid mentioning Tor2Web mode as an alternative. (But I think we can keep the code as-is, it will work for the hidden service client case, but not the Exit case.)