wgengine/monitor: fix docs on Mon.InterfaceState

The behavior was changed in March (in 7f174e84e6)
but that change forgot to update these docs.

Change-Id: I79c0301692c1d13a4a26641cc5144baf48ec1360
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/monitor/monitor.go

@@ -104,8 +104,10 @@ func New(logf logger.Logf) (*Mon, error) {
 	return m, nil
 }
 
-// InterfaceState returns the state of the machine's network interfaces,
+// InterfaceState returns the latest snapshot of the machine's network
-// without any Tailscale ones.
+// interfaces.
+//
+// The returned value is owned by Mon; it must not be modified.
 func (m *Mon) InterfaceState() *interfaces.State {
 	m.mu.Lock()
 	defer m.mu.Unlock()