doc(stream): add docs on initializing the replicated state table (#17027

)

docs/backfill.md

@@ -358,6 +358,22 @@ and arrangement backfill will consume this historical data snapshot:
 | 1                  | 'Jack' | 29 |
 | 2                  | 'Jill' | 30 |
 
+#### Initialization
+
+Something to note is that for the first snapshot,
+upstream may not have finished committing data in that epoch to s3.
+
+Additionally, we have not replicated any upstream records
+during that epoch, only in the subsequent ones.
+
+As such, we must wait for that first checkpoint to be committed,
+before reading, or we risk missing the uncommitted data in our backfill.
+
+This is supported internally inside `init_epoch` for replicated state table.
+```shell
+        upstream_table.init_epoch(first_epoch).await?;
+```
+
 ### Recovery
 
 TODO