cherry-pick: fix docs describing handling of empty commits

cherry-pick: fix docs describing handling of empty commits

Commit b27cfb0 (git-cherry-pick: Add keep-redundant-commits
option, 2012-04-20), added the --keep-redundant-commits
option, and switched the default behavior (without that
option) to silently ignore empty commits. Later, the second
half of that commit was reverted in ac2b0e8 (cherry-pick:
regression fix for empty commits, 2012-05-29), but the
documentation added for --keep-redundant-commits was never
updated to match. Let's do so now.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>