docs: dev: topology-over-raft: document the left_token_ring state

In one of the previous patches, we changed the `left_token_ring`
state from a node state to a transition state. We document it
in this patch. The node state wasn't documented, so there is
nothing to remove.

docs/dev/topology-over-raft.md

@@ -37,6 +37,9 @@ Additionally to specific node states, there entire topology can also be in a tra
     Writes are going to both new and old replicas (new replicas means calculated according to modified
 token ring), reads are using old replicas.
 - `write_both_read_new` - as above, but reads are using new replicas.
+- `left_token_ring` - the decommissioning node left the token ring, but we still need to wait until other
+   nodes observe it and stop sending writes to this node. Then, we tell the node to shut down and remove
+   it from group 0. We also use this state to rollback a failed bootstrap or decommission.
 
 When a node bootstraps, we create new tokens for it and a new CDC generation
 and enter the `commit_cdc_generation` state. Once the generation is committed,