Documentation/gc: warn against --prune=<now>

Documentation/gc: warn against --prune=<now>

"git gc" is safe to run anytime only because it has the built-in
grace period to protect objects that are created by other processes
that are waiting for ref updates to anchor them to the history.  In
order to run with no grace period, the user must make sure that the
repository is quiescent.

Reviewed-by: Matthieu Moy <Matthieu.Moy@grenoble-inp.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>