dconf: note that system dconf must be enabled

Before enabling dconf in Home Manager, dconf must be enabled in system
config.

Otherwise it will fail like this:

```
$ home-manager switch
Starting Home Manager activation
Activating checkFilesChanged
Activating checkLinkTargets
Activating writeBoundary
Activating installPackages
replacing old 'home-manager-path'
installing 'home-manager-path'
Activating dconfSettings
error: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit dconf.service not found.
```

docs/faq.adoc

@@ -91,14 +91,22 @@ while the `common.nix` file contains configuration shared across the two logins.
 
 You can get some inspiration from the {post-your-homenix}[Post your home-manager home.nix file!] Reddit thread.
 
-=== Why do I get an error message about `ca.desrt.dconf`?
+=== Why do I get an error message about `ca.desrt.dconf` or `dconf.service`?
 
-You are most likely trying to configure the GTK or Gnome Terminal but the DBus session is not aware of the dconf service. The full error you might get is
+You are most likely trying to configure something that uses dconf
+but the DBus session is not aware of the dconf service.
+The full error you might get is
 
 ----
 error: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The name ca.desrt.dconf was not provided by any .service files
 ----
 
+or
+
+----
+error: GDBus.Error:org.freedesktop.systemd1.NoSuchUnit: Unit dconf.service not found.
+----
+
 The solution on NixOS is to add
 
 [source,nix]

modules/misc/dconf.nix

@@ -21,6 +21,11 @@ in {
         visible = false;
         description = ''
           Whether to enable dconf settings.
+          </para><para>
+          Note, if you use NixOS then you must add
+          <code>programs.dconf.enable = true</code>
+          to your system configuration. Otherwise you will see a systemd error
+          message when your configuration is activated.
         '';
       };