config doc: elaborate on fetch.fsckObjects security

config doc: elaborate on fetch.fsckObjects security

Change the transfer.fsckObjects documentation to explicitly note the
unique security and/or corruption issues fetch.fsckObjects suffers
from, since it doesn't have a quarantine environment.

This was already alluded to in the existing documentation, but let's
spell it out so there's no confusion here, and give a concrete example
of how to work around this limitation.

Let's also prominently note that this is considered to be a limitation
of the current implementation, rather than something that's intended
and by design, since we might change this in the future.

See
https://public-inbox.org/git/20180531060259.GE17344@sigill.intra.peff.net/
for further details.

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