Mention upgrade migrate job error (#2422)

* Mention upgrade migrate job error

* nit

contents/docs/self-host/deploy/snippets/upgrading.mdx

@@ -20,3 +20,15 @@ Compare the numbers for the chart version (in the format `posthog-{major}.{minor
 <CommandHelmUpgradeSnippet />
 
 Check the Helm [documentation](https://helm.sh/docs/helm/helm_upgrade/) for more info about the `helm upgrade` command.
+
+<details>
+<summary>Troubleshooting</summary>
+If you see this error
+
+```
+Error: UPGRADE FAILED: release posthog failed, and has been rolled back due to atomic being set: post-upgrade hooks failed: warning: Hook post-upgrade posthog/templates/migrate.job.yaml failed: jobs.batch "posthog-migrate" already exists
+```
+
+it happens because the migrate job was left around from a previous upgrade attempt, we just need to kill that job (`kubectl delete job posthog-migrate -n posthog`) before running the upgrade again.
+</details>
+