tor_addr_is_internal should never block PT dummy addresses

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

In legacy/trac#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).

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