Warn at startup when worker or cascade limits are far past the cores

More than four times os.cpu_count() logs one warning; the value is kept.

docs/reference/configuration.md

@@ -137,7 +137,9 @@ Set `TZ` in `.env` for schedules that should follow local time, such as
 | `OBS_RETENTION_DAYS` | `30` | retention for metrics, events and run records |
 
 The three concurrency limits must be at least `1`. They also have CLI flags:
-`--max-workers`, `--max-cascades` and `--max-runs`. Resource flags are `--gpus`,
+`--max-workers`, `--max-cascades` and `--max-runs`. When `FLOW_MAX_WORKERS` or
+`FLOW_MAX_CASCADES` is more than four times the machine's core count, the
+engine logs a warning at startup and uses the value anyway. Resource flags are `--gpus`,
 `--ram` and `--worker-max-rss`.
 
 Nodes with declared resources are scheduled against the available budget.