documentation update on postStart hook use

Author: k-popov

docs/source/jupyterhub/customizing/user-environment.md

@@ -331,6 +331,12 @@ container, meaning that commands that place files relative to `./` will result
 in users seeing those files in their home directory. You can use commands like
 `wget` to place files where you like.
 
+It's also worth mentioning that Kubernetes PostStart hook is
+[executed asynchronously](https://kubernetes.io/docs/concepts/containers/container-lifecycle-hooks/#hook-handler-execution).
+This means there is no guarantee that the hook will execute before the container
+ENTRYPOINT. This makes postStart hook unusable if some initialization must complete
+before the user server start. For these cases a [custom command](#set-cmd) may be used.
+
 A simple way to populate the notebook user's home directory is to add the
 required files to the container's `/tmp` directory and then copy them to
 `/home/jovyan` using a `postStart` hook. This example shows the use of