add proxmox_delegate_host variable and update check-exists documentation

- Document proxmox_delegate_host for delegating pct commands to inventory host
- Update check-exists.yml description to reflect vmid or hostname checking
- Document lxc_exists fact set by idempotency check

roles/proxmox_lxc_provision/defaults/main.yml

@@ -2,6 +2,8 @@
 # Proxmox API connection defaults (optional)
 proxmox_api_port: 8006
 proxmox_api_validate_certs: false
+# Host to delegate pct commands to (use inventory hostname for become_password to work)
+proxmox_delegate_host: "{{ proxmox_api_host }}"
 
 # LXC defaults
 lxc_template: "local:vztmpl/debian-12-standard_12.12-1_amd64.tar.zst"