docs(roles/hetzner_vm): explain how to resize disk

Author: bhatti-lf

roles/hetzner_vm/README.md

@@ -260,6 +260,11 @@ hetzner_vm__volumes:
 ```
 
 
+## Troubleshooting
+
+* When a small VM is later rescaled to a larger one, the disk size stays the same. This is intentional: Hetzner disks cannot be shrunk, so growing the disk would block any later scale-down. To resize the disk anyway, scale the VM back down, set `hetzner_vm__upgrade_disk` to `true`, and then scale it back up.
+
+
 ## License
 
 [The Unlicense](https://unlicense.org/)