ART: Add comments to dex2oat return codes

Add documentation in comments. Follow-up to changes
7b5659113309e8db2558ae72870b67ebdeb7233c and
f788297b28e0c91945ae5f4d41f987514ce45e6f.

Bug: 36336122
Test: m
Change-Id: If1188fc21a337dd7689361d13fe00434c15d3202
1 file changed