Document the cascade guarantee and the cascade ceiling

A node runs when something it reads was published, not merely when something
upstream of it ran — worth saying now that it is true, since it is what a
reader relies on when a rate-limited port goes quiet. Plus `FLOW_MAX_CASCADES`
in the engine's configuration table.

docs/reference/configuration.md

@@ -123,6 +123,7 @@ warning into a refusal to start.
 | Variable | Default | Notes |
 |---|---|---|
 | `FLOW_MAX_WORKERS` | `4` | node-code subprocesses run in parallel |
+| `FLOW_MAX_CASCADES` | `4` | cascades in flight at once; throughput is this over the mean cascade time, so raise it where nodes wait on a network rather than a CPU |
 | `FLOW_NODE_TIMEOUT` | `0` | seconds a node may be silent, unless it sets its own; 0 is no limit |
 | `OBS_RETENTION_DAYS` | `30` | how long metrics, events and run records are kept |