docs(nns): Note that system subnets cannot be deleted in DeleteSubnet description

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: mraszyk

rs/nns/governance/api/src/types.rs

@@ -4285,6 +4285,7 @@ pub enum NnsFunction {
     /// Delete a subnet. The subnet record, catch-up package, threshold signing key
     /// and routing table entries are removed from the registry, the subnet is
     /// removed from the subnet list, and the subnet's nodes become unassigned.
+    /// System subnets (e.g. the NNS or II subnet) cannot be deleted.
     DeleteSubnet = 57,
     /// Set or unset the default subnet to which `SetupInitialDKG` management
     /// canister calls are routed when no subnet is specified explicitly. If unset,

rs/nns/governance/proto/ic_nns_governance/pb/v1/governance.proto

@@ -503,6 +503,7 @@ enum NnsFunction {
   // Delete a subnet. The subnet record, catch-up package, threshold signing key
   // and routing table entries are removed from the registry, the subnet is
   // removed from the subnet list, and the subnet's nodes become unassigned.
+  // System subnets (e.g. the NNS or II subnet) cannot be deleted.
   NNS_FUNCTION_DELETE_SUBNET = 57;
 
   // Set or unset the default subnet to which `SetupInitialDKG` management

rs/nns/governance/src/gen/ic_nns_governance.pb.v1.rs

@@ -5279,6 +5279,7 @@ pub enum NnsFunction {
     /// Delete a subnet. The subnet record, catch-up package, threshold signing key
     /// and routing table entries are removed from the registry, the subnet is
     /// removed from the subnet list, and the subnet's nodes become unassigned.
+    /// System subnets (e.g. the NNS or II subnet) cannot be deleted.
     DeleteSubnet = 57,
     /// Set or unset the default subnet to which `SetupInitialDKG` management
     /// canister calls are routed when no subnet is specified explicitly. If unset,

rs/nns/governance/src/proposals/execute_nns_function.rs

@@ -936,7 +936,8 @@ impl ValidNnsFunction {
             ValidNnsFunction::DeleteSubnet => {
                 "Delete a subnet. The subnet record, catch-up package, threshold signing key \
                 and routing table entries are removed from the registry, the subnet is \
-                removed from the subnet list, and the subnet's nodes become unassigned."
+                removed from the subnet list, and the subnet's nodes become unassigned. \
+                System subnets (e.g. the NNS or II subnet) cannot be deleted."
             }
             ValidNnsFunction::SetDefaultInitialDkgSubnet => {
                 "Set or unset the default subnet to which `SetupInitialDKG` management canister \