Documentation/git-multi-pack-index.txt: clarify expire behavior
The `expire` sub-command of `git multi-pack-index` will never expire
`.keep` packs, regardless of whether or not any of their objects were
selected in the MIDX.
This has always been the case since
19575c7c8e (multi-pack-index:
implement 'expire' subcommand, 2019-06-10), which came after
cff9711616
(multi-pack-index: prepare for 'expire' subcommand, 2019-06-10), when
this documentation was originally written.
Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>