Jeff King authored May 28, 2018 and Junio C Hamano committed May 29, 2018

Commit 32637cdf

 (show-index.c: learn about index v2,
2007-04-09) changed the output format of show-index to
include the object CRC32 but didn't update the
documentation. Let's fix that and generally describe the
output in more detail.

There are a few other fixes here while we're rewording:

 - refer to index-pack along with pack-objects, since either
   can create .idx files

 - use "linkgit:" for referring to other commands

 - expand the bit about verify-pack, giving reasons why you
   might want this command instead. I almost omitted this
   entirely, but referring to verify-pack might help a
   reader who is looking for more information.

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