nixos/doc: explain how to use the ff sync module with ff android (#413380)

Author: Aleksanaa

nixos/doc/manual/redirects.json

@@ -50,6 +50,15 @@
   "module-services-crab-hole-upstream-options": [
     "index.html#module-services-crab-hole-upstream-options"
   ],
+  "module-services-firefox-syncserver-clients": [
+    "index.html#module-services-firefox-syncserver-clients"
+  ],
+  "module-services-firefox-syncserver-clients-android": [
+    "index.html#module-services-firefox-syncserver-clients-android"
+  ],
+  "module-services-firefox-syncserver-clients-desktop": [
+    "index.html#module-services-firefox-syncserver-clients-desktop"
+  ],
   "module-services-opencloud": [
     "index.html#module-services-opencloud"
   ],

nixos/modules/services/networking/firefox-syncserver.md

@@ -24,10 +24,8 @@ The absolute minimal configuration for the sync server looks like this:
 }
 ```
 
-This will start a sync server that is only accessible locally. Once the services is
-running you can navigate to `about:config` in your Firefox profile and set
-`identity.sync.tokenserver.uri` to `http://localhost:5000/1.0/sync/1.5`. Your browser
-will now use your local sync server for data storage.
+This will start a sync server that is only accessible locally on the following url: `http://localhost:5000/1.0/sync/1.5`.
+See [the dedicated section](#module-services-firefox-syncserver-clients) to configure your browser to use this sync server.
 
 ::: {.warning}
 This configuration should never be used in production. It is not encrypted and
@@ -55,3 +53,25 @@ be made via TLS.
 
 For actual deployment it is also recommended to store the `secrets` file in a
 secure location.
+
+## Configuring clients to use this server {#module-services-firefox-syncserver-clients}
+
+### Firefox desktop {#module-services-firefox-syncserver-clients-desktop}
+To configure a desktop version of Firefox to use your server, navigate to
+`about:config` in your Firefox profile and set
+`identity.sync.tokenserver.uri` to `https://myhostname:5000/1.0/sync/1.5`.
+
+### Firefox Android {#module-services-firefox-syncserver-clients-android}
+To configure an Android version of Firefox to use your server:
+* First ensure that you are disconnected from you Mozilla account.
+* Go to App Menu > Settings > About Firefox and click the logo 5 times. You
+  should see a “debug menu enabled” notification.
+* Back to the main menu, a new menu "sync debug" should have appeared.
+* In this menu, set "custom sync server" to `https://myhostname:5000/1.0/sync/1.5`.
+
+::: {.warning}
+Changes to this configuration value are ignored if you are currently connected to your account.
+:::
+
+* Restart the application.
+* Log in to your account.