Clarify promtail troubleshooting around positions (#351)

docs/troubleshooting.md

@@ -6,10 +6,10 @@ This error can appear in Grafana when you add Loki as a datasource.
 It means that Grafana can connect to Loki, but Loki has not received any logs from promtail.
 This can have several reasons:
 
-- Promtail cannot reach Loki, check promtails output.
-- Promtail started sending logs before Loki was ready. This can happen in test environments where promtail already read all logs and sent them off. You have two options:
-  - Start promtail later, e.g., 60 seconds
-  - Make sure new log messages are written after both promtail and Loki have started
+- Promtail cannot reach Loki, check promtail's output.
+- Promtail started sending logs before Loki was ready. This can happen in test environments where promtail already read all logs and sent them off. Here is what you can do:
+  - Generally start promtail after Loki, e.g., 60 seconds later.
+  - Restarting promtail will not necessarily resend log messages that have been read. To force sending all messages again, delete the positions file (default location `/tmp/positions.yaml`) or make sure new log messages are written after both promtail and Loki have started.
 
 ## Failed to create target, "ioutil.ReadDir: readdirent: not a directory"