Include example Prometheus command line (#884)

Make it more clear what the Prometheus command line flags should be for sidecar use.

docs/components/sidecar.md

@@ -13,7 +13,14 @@ Prometheus servers connected to the Thanos cluster via the sidecar are subject t
 The retention is recommended to not be lower than three times the block duration. This achieves resilience in the face of connectivity issues 
 to the object storage since all local data will remain available within the Thanos cluster. If connectivity gets restored the backlog of blocks gets uploaded to the object storage.
 
+```console
+$ prometheus \
+  --storage.tsdb.max-block-duration=2h \
+  --storage.tsdb.min-block-duration=2h \
+  --web.enable-lifecycle
 ```
+
+```console
 $ thanos sidecar \
     --tsdb.path        "/path/to/prometheus/data/dir" \
     --prometheus.url   "http://localhost:9090" \