Update local-pv documentation (#383)

* Update local-pv documentation

Signed-off-by: Aylei <rayingecho@gmail.com>

docs/setup.md

@@ -91,13 +91,22 @@ Use this command to confirm the mount point exist:
 $ mount | grep /mnt/disks/local-pv01
 ```
 
-To auto-mount disks when your operating system is booted, you should edit `/etc/fstab` to include these mounting info:
+### Auto-mount on reboot
 
+To auto-mount disks when your operating system is booted, you should edit `/etc/fstab` to include these mounting info.
+
+Disk mount:
+```shell
+$ echo "/dev/nvme0n1 /mnt/disks/disk01 ext4 defaults 0 0" >> /etc/fstab
+```
+
+Bind mount:
 ```shell
-$ echo "/dev/nvme0n1 /mnt/disks/disk0 none bind 0 0" >> /etc/fstab
 $ echo "/data/local-pv01 /mnt/disks/local-pv01 none bind 0 0" >> /etc/fstab
 ```
 
+### Deploy local-static-provisioner
+
 After mounting all data disks on Kubernetes nodes, you can deploy [local-volume-provisioner](https://github.com/kubernetes-sigs/sig-storage-local-static-provisioner) to automatically provision the mounted disks as Local PersistentVolumes.
 
 ```shell
@@ -106,6 +115,20 @@ $ kubectl get po -n kube-system -l app=local-volume-provisioner
 $ kubectl get pv | grep local-storage
 ```
 
+### Remove a mount point
+
+If we want to remove a mount point, first we need to `umount` the mount point, and then delete the related directories. For example:
+
+```shell
+$ umount /mnt/disks/local-pv01
+$ rm -rf /mnt/disks/local-pv01 
+$ rm -rf /data/local-pv01
+```
+
+You should also delete the related entries in `/etc/fstab` at the same time, otherwise the automount may cause problems when the machine restarts.
+
+> Note: The local-volume plugin expects paths to be stable, if you remove a previous mount-point in the discovery directory (default to `/mnt/disks/`), you should remove the PV manually to keep consistency.
+
 ## Install TiDB Operator
 
 TiDB Operator uses [CRD](https://kubernetes.io/docs/tasks/access-kubernetes-api/custom-resources/custom-resource-definitions/) to extend Kubernetes, so to use TiDB Operator, you should first create `TidbCluster` custom resource kind. This is a one-time job, namely you can only need to do this once in your Kubernetes cluster.