Document which interface is used for DNS requests in the context of OutboundBindAddressOR/Exit

In legacy/trac#17975 (moved) two new options have been added:

The manual page makes no clear statement which interface is used for DNS traffic.

https://trac.torproject.org/projects/tor/ticket/17975#comment:13 hints towards OutboundBindAddressOR, but users should not be required to search the bugtracker find out about that.

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