commit-graph: update design document

commit-graph: update design document

As it exists right now, the commit-graph feature may provide
inconsistent results when combined with commit grafts, replace objects,
and shallow clones. Update the design document to discuss why these
interactions are difficult to reconcile and how we will avoid errors by
preventing updates to and reads from the commit-graph file when these
other features exist.

Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>