Add some more details about useSandbox

nixos/modules/services/misc/nix-daemon.nix

@@ -105,7 +105,9 @@ in
           If set, Nix will perform builds in a sandboxed environment that it
           will set up automatically for each build.  This prevents
           impurities in builds by disallowing access to dependencies
-          outside of the Nix store.
+          outside of the Nix store. This isn't enabled by default for
+          performance. It doesn't affect derivation hashes, so changing
+          this option will not trigger a rebuild of packages.
         ";
       };