Convert update-index references in docs to add. Since `git add` is the approved porcelain for an end-user to invoke when they want to manipulate the index, porcelain documentation should steer the user to this command rather than the pure plumbing update-index. Signed-off-by: Shawn O. Pearce <spearce@spearce.org> Signed-off-by: Junio C Hamano <junkio@cox.net>