Bisect: add "bisect skip" to the documentation. Also fix "bisect bad" and "bisect good" short usage description. Signed-off-by: Christian Couder <chriscool@tuxfamily.org> Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>