config doc: elaborate on what transfer.fsckObjects does

config doc: elaborate on what transfer.fsckObjects does

The existing documentation led the user to believe that all we were
doing were basic reachability sanity checks, but that hasn't been true
for a very long time. Update the description to match reality, and
note the caveat that there's a quarantine for accepting pushes, but
not for fetching.

Also mention that the fsck checks for security issues, which was my
initial motivation for writing this fetch.fsck.* series.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>