docs(nix): document home-manager activation error

by default home-manager depend on files under ~/.nix-profile, which is
not ideal

flake.nix

@@ -77,6 +77,10 @@
       };
 
       darwinConfigurations = mkSystem {
+        # NOTE: home manager activation is showing following error
+        # error: profile '/Users/william/.local/state/nix/profiles/profile' is incompatible with 'nix-env'; please use 'nix profile' instead
+        # temporary workaround is to synlink ~/.local/state/nix/profile to ~/.nix-profile
+        # ref: https://discourse.nixos.org/t/home-manager-insists-on-using-nix-profile/57708
         isDarwin = true;
       };
 

lib/nix.nix

@@ -17,6 +17,7 @@ in
       auto-optimise-store = !pkgs.stdenv.isDarwin;
 
       # add these to global /etc/nix/nix.conf when using home manager, and restart nix-daemon by `systemctl restart nix-daemon`
+      # nix-daemon doesn't watch the nix.conf file: https://github.com/NixOS/nix/issues/8939
       trusted-users = [
         "root"
         "@wheel"