tor_addr_is_internal should never block PT dummy addresses

In #17674 (moved) and #8976 (moved), we block internal addresses when opening a connection.

In #18517 (moved), we chose 192.0.2.0/24 as a dummy address range for Tor Browser to use for pluggable transports to use when they don't have the actual remote address.

Let's add a comment near the IPv4 range in tor_addr_is_internal_ to never block those addresses (or, if we do, to change how Tor Browser and PTs work first).