docs: clarify nebula.overlayCidr as reserved for future use

Value is defined but not referenced in any template. Nebula derives
overlay IPs from certificates, not config. Retained for planned
networkpolicy/tun configuration with updated comment.

Author: privsim

helm/disentangle/values.yaml

@@ -106,7 +106,7 @@ nebula:
     pullPolicy: IfNotPresent
   # Mode: "lighthouse" or "node"
   mode: "node"
-  # Overlay network CIDR (e.g., 10.42.0.0/16)
+  # Overlay network CIDR - reserved for future networkpolicy/tun configuration
   overlayCidr: "10.42.0.0/16"
   # Lighthouse address (ip:port) - required for mode=node
   lighthouseAddr: ""