[FLINK-15999][doc] Add TODOs to state concepts section

docs/concepts/stateful-stream-processing.md

@@ -317,6 +317,10 @@ updates to that state.
 See [Restart Strategies]({{ site.baseurl }}{% link dev/task_failure_recovery.md
 %}#restart-strategies) for more information.
 
+### State Backends
+
+`TODO: add`
+
 ### Exactly Once vs. At Least Once
 
 The alignment step may add latency to the streaming program. Usually, this
@@ -340,9 +344,12 @@ repartitioning/shuffle). Because of that, dataflows with only embarrassingly
 parallel streaming operations (`map()`, `flatMap()`, `filter()`, ...) actually
 give *exactly once* guarantees even in *at least once* mode.
 
-
 {% top %}
 
+## End-to-end Exactly-Once Programs
+
+`TODO: add`
+
 ## State and Fault Tolerance in Batch Programs
 
 Flink executes [batch programs](../dev/batch/index.html) as a special case of