Update docs/maintenance-checking-services.md and docs/quick-start.md: add instruction to use federation tester against the base domain

Per Slavi's suggestion.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

docs/maintenance-checking-services.md

@@ -10,4 +10,4 @@ ansible-playbook -i inventory/hosts setup.yml --tags=self-check
 
 If it's all green, everything is probably running correctly.
 
-Besides this self-check, you can also check your server using the [Federation Tester](https://federationtester.matrix.org/).
+Besides this self-check, you can also check whether your server federates with the Matrix network by using the [Federation Tester](https://federationtester.matrix.org/) against your base domain (`example.com`), not the `matrix.example.com` subdomain.

docs/quick-start.md

@@ -171,7 +171,7 @@ After configuring the playbook, run the installation command and wait until it f
 ansible-playbook -i inventory/hosts setup.yml --tags=install-all,start
 ```
 
-After the command finishes, you can also check whether your server federates with the Matrix network, by using the [Federation Tester](https://federationtester.matrix.org/).
+After the command finishes, you can also check whether your server federates with the Matrix network by using the [Federation Tester](https://federationtester.matrix.org/) against your base domain (`example.com`), not the `matrix.example.com` subdomain.
 
 If you think something is off with the server configuration, feel free to re-run the setup command any time.