For shards which have 1 replica ( leader ) we know it doesn't need to recover from anyone. We should short-circuit the recovery process in this case.
The motivation for this being that we will generate less state events and be able to mark these replicas as active again without it needing to go into 'recovering' state.
We already short circuit when you set -Dsolrcloud.skip.autorecovery=true but that sys prop was meant for tests only. Extending this to make sure the code short-circuits when the core knows its the only replica in the shard is the motivation of the Jira.