[#4611] put project_id first in Shortlink index; add comments on index usage
[#4611] put project_id first in Shortlink index; add comments on index usage

apache/allura,heiths/allura,apache/incubator-allura,apache/incubator-allura,lym/allura-git,heiths/allura,lym/allura-git,apache/allura,Bitergia/allura,lym/allura-git,apache/incubator-allura,lym/allura-git,apache/allura,apache/incubator-allura,Bitergia/allura,lym/allura-git,heiths/allura,apache/allura,heiths/allura,Bitergia/allura,Bitergia/allura,heiths/allura,apache/allura,Bitergia/allura