provider/openstack: Add documentation about user_data (#14411)

website/source/docs/providers/openstack/r/compute_instance_v2.html.markdown

@@ -259,6 +259,26 @@ resource "openstack_compute_instance_v2" "multi-eph" {
 }
 ```
 
+### Instance with User Data (cloud-init)
+
+```hcl
+resource "openstack_compute_instance_v2" "instance_1" {
+  name            = "basic"
+  image_id        = "ad091b52-742f-469e-8f3c-fd81cadf0743"
+  flavor_id       = "3"
+  key_pair        = "my_key_pair_name"
+  security_groups = ["default"]
+  user_data       = "#cloud-config\nhostname: instance_1.example.com\nfqdn: instance_1.example.com"
+
+  network {
+    name = "my_network"
+  }
+}
+```
+
+`user_data` can come from a variety of sources: inline, read in from the `file`
+function, or the `template_cloudinit_config` resource.
+
 ## Argument Reference
 
 The following arguments are supported: