Update docs/configuring-playbook-bridge-mautrix-bridges.md: add description about the relaybot

Based on:
- the official documentation: https://docs.mau.fi/bridges/general/relay-mode.html
- docs/configuring-playbook-bridge-mautrix-whatsapp.md

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

docs/configuring-playbook-bridge-mautrix-bridges.md

@@ -60,7 +60,9 @@ matrix_mautrix_SERVICENAME_bridge_encryption_default: true
 
 ### Enable relay mode (optional)
 
-[Relay mode](https://docs.mau.fi/bridges/general/relay-mode.html) is off by default. If you would like to enable relay mode, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file:
+[Relay mode](https://docs.mau.fi/bridges/general/relay-mode.html) is off by default. Check [the table on the official documentation](https://docs.mau.fi/bridges/general/relay-mode.html#support-table) for bridges which support relay mode.
+
+If you would like to enable it, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file:
 
 **for all bridges with relay mode support**:
 
@@ -89,6 +91,18 @@ matrix_mautrix_SERVICENAME_configuration_extension_yaml: |
       default: true
 ```
 
+If you want to activate the relaybot in a room, send `!prefix set-relay` in the rooms where you want to use the bot (replace `!prefix` with the appropriate command prefix for the bridge, like `!signal` or `!wa`). To deactivate, send `!prefix unset-relay`.
+
+Use `!prefix set-pl 100` to be able for the bot to modify room settings and invite others.
+
+#### Allow anyone on the homeserver to become a relay user (optional)
+
+By default, only admins are allowed to set themselves as relay users. To allow anyone on your homeserver to set themselves as relay users, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file:
+
+```yaml
+matrix_mautrix_SERVICENAME_bridge_relay_admin_only: false
+```
+
 ### Set the bot's username (optional)
 
 To set the bot's username, add the following configuration to your `inventory/host_vars/matrix.example.com/vars.yml` file: