docs: update offset order for pack index v3

docs: update offset order for pack index v3

The current design of pack index v3 has items in two different orders:
sorted shortened object ID order and pack order.  The shortened object
IDs and the pack index offset values are in the former order and
everything else is in the latter.

This, however, poses some problems.  We have many parts of the packfile
code that expect to find out data about an object knowing only its index
in pack order.  With the current design, to find the pack offset after
having looked up the index in pack order, we must then look up the full
object ID and use that to look up the shortened object ID to find the
pack offset, which is inconvenient, inefficient, and leads to poor cache
usage.

Instead, let's change the offset values to be looked up by pack order.
This works better because once we know the pack order offset, we can
find the full object name and its location in the pack with a simple
index into their respective tables.  This makes many operations much
more efficient, especially with the functions we already have, and it
avoids the need for the revindex with pack index v3.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>