prereqs for regional disks (hashicorp#1753)

In testing an upcoming `google_compute_region_disk` resource, I had to make these changes. Checking them in separately so that when the magician runs, these changes will already be a part of TF.

Author: danawillow

docs/r/compute_instance_template.html.markdown

@@ -206,7 +206,8 @@ The `disk` block supports:
     read-write mode.
 
 * `source` - (Required if source_image not set) The name of the disk (such as
-    those managed by `google_compute_disk`) to attach.
+    those managed by `google_compute_disk`) to attach. This cannot be a regional
+    disk.
 
 * `disk_type` - (Optional) The GCE disk type. Can be either `"pd-ssd"`,
     `"local-ssd"`, or `"pd-standard"`.