nixos/hostapd: document that legacy example should have optional MFP

Thinkpad x230, for example, is unable to connect to the legacy example
if managementFrameProtection is required.

nixos/modules/services/networking/hostapd.nix

@@ -161,6 +161,7 @@ in {
                   mode = "wpa2-sha256";
                   wpaPassword = "a flakey password"; # Use wpaPasswordFile if possible.
                 };
+                managementFrameProtection = "optional";
               };
             };
           }