Update mig.md

Author: jovial

docs/mig.md

@@ -151,7 +151,17 @@ Use the ``vgpu`` metadata option to enable creation of mig devices on rebuild.
 
 ## GRES configuration
 
-You should stop terraform templating out partitions.yml and specify `openhpc_nodegroups` manually.
+You should stop terraform templating out partitions.yml and specify `openhpc_nodegroups` manually. To do this
+set the `autogenerated_partitions_enabled` terraform variable to `false`. For example (`environments/production/tofu/main.tf`):
+
+```
+module "cluster" {
+  source = "../../site/tofu/"
+  ...
+  # We manually populate this to add GRES. See environments/site/inventory/group_vars/all/partitions-manual.yml.
+  autogenerated_partitions_enabled = false
+}
+```
 
 GPU types can be determined by deploying slurm without any gres configuration and then running
 `sudo slurmd -G` on a compute node where GPU resources exist. An example is shown below: