Convert update-index references in docs to add.

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>