docs: show distinct_hosts constraint for CSI plugins (hashicorp#9052)

CSI plugins with the same plugin ID and type (controller, node, monolith) will
collide on a host, both in the communication socket and in the dynamic plugin
registry. Until this can be fixed, leave notice to operators in the
documentation.

website/pages/docs/job-specification/csi_plugin.mdx

@@ -56,6 +56,11 @@ the `node` or `monolith` plugin for those volumes. You should run `node` or
 flag on `nomad node drain` to ensure that the plugins are running while the
 node is being drained.
 
+~> **Note:** Only one plugin instance of a given plugin ID and type
+(controller or node) should be deployed on any given client node. Use a
+constraint as shown below.
+
+
 ## `csi_plugin` Examples
 
 ```hcl
@@ -66,6 +71,13 @@ job "plugin-efs" {
   # as a system job ensures all nodes in the DC have a copy.
   type = "system"
 
+  # only one plugin of a given type and ID should be deployed on
+  # any given client node
+  constraint {
+    operator = "distinct_hosts"
+    value = true
+  }
+
   group "nodes" {
     task "plugin" {
       driver = "docker"