pf.conf.5: improve reply-to documentation

pf.conf.5: improve reply-to documentation

reply-to uses addresses, not interfaces anymore since
https://marc.info/?l=openbsd-cvs&m=161213948819452&w=2

make it clearer that reply-to allows for symmetric routing enforcement,
eg replying via a specific gateway when having multiple paths.

wording from sthen@, vastly improving my initial suggestion.
ok jmc@ dlg@