common: document ghobject sort order rationale

Intuition differs regarding the sort order of the ghobject shard and
generation. Document the rationale for the chosen sort order.

Reviewed-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: David Zafman <david.zafman@inktank.com>
Signed-off-by: Loic Dachary <loic@dachary.org>