Update documentation for components which require subdomain settings by default

This adopts the structure of docs/configuring-playbook-matrix-authentication-service.md which was recently created.

- …
- Adjusting the playbook configuration
  - …
  - Adjusting the (service name here) URL
  - …
- Adjusting DNS records
- Installing
- …

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

docs/configuring-playbook-client-element.md

@@ -5,14 +5,14 @@ By default, this playbook installs the [Element](https://github.com/element-hq/e
 
 ## Disabling Element
 
-If you'd like for the playbook to not install Element (or to uninstall it if it was previously installed), you can disable it in your configuration file (`inventory/host_vars/matrix.example.com/vars.yml`):
+If you'd like for the playbook to not install Element (or to uninstall it if it was previously installed), add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file:
 
 ```yaml
 matrix_client_element_enabled: false
 ```
 
 
-## Configuring Element settings
+## Adjusting the playbook configuration
 
 The playbook provides some customization variables you could use to change Element's settings.
 
@@ -29,7 +29,7 @@ Alternatively, **if there is no pre-defined variable** for an Element setting yo
 - or, if extending the configuration is still not powerful enough for your needs, you can **override the configuration completely** using `matrix_client_element_configuration_default` (or `matrix_client_element_configuration`). You can find information about this in [`roles/custom/matrix-client-element/defaults/main.yml`](../roles/custom/matrix-client-element/defaults/main.yml).
 
 
-## Themes
+### Themes
 
 To change the look of Element, you can define your own themes manually by using the `matrix_client_element_setting_defaults_custom_themes` setting.
 
@@ -38,3 +38,32 @@ Or better yet, you can automatically pull it all themes provided by the [aaronra
 If you make your own theme, we encourage you to submit it to the **aaronraimist/element-themes** project, so that the whole community could easily enjoy it.
 
 Note that for a custom theme to work well, all Element instances that you use must have the same theme installed.
+
+### Adjusting the Element URL
+
+By default, this playbook installs Element on the `element.` subdomain (`element.example.com`) and requires you to [adjust your DNS records](#adjusting-dns-records).
+
+By tweaking the `matrix_client_element_hostname` and `matrix_client_element_path_prefix` variables, you can easily make the service available at a **different hostname and/or path** than the default one.
+
+Example additional configuration for your `inventory/host_vars/matrix.example.com/vars.yml` file:
+
+```yaml
+# Switch to the domain used for Matrix services (`matrix.example.com`),
+# so we won't need to add additional DNS records for Element.
+matrix_client_element_hostname: "{{ matrix_server_fqn_matrix }}"
+
+# Expose under the /element subpath
+matrix_client_element_path_prefix: /element
+```
+
+## Adjusting DNS records
+
+Once you've decided on the domain and path, **you may need to adjust your DNS** records to point the Element domain to the Matrix server.
+
+By default, you will need to create a CNAME record for `element`. See [Configuring DNS](configuring-dns.md) for details about DNS changes.
+
+If you've decided to reuse the `matrix.` domain, you won't need to do any extra DNS configuration.
+
+## Installing
+
+After configuring the playbook, run the [installation](installing.md) command: `just install-all` or `just setup-all`