Add comments around rtm

Comments taken from https://github.com/matrix-org/matrix-appservice-slack/blob/develop/config/config.sample.yaml

We should probably reconcile our configuration with that one and include comments for other fields as well.

roles/matrix-bridge-appservice-slack/templates/config.yaml.j2

@@ -9,8 +9,20 @@ homeserver:
     url: "{{ matrix_appservice_slack_homeserver_url }}"
     media_url: "{{ matrix_appservice_slack_homeserver_media_url }}"
 
+# Real Time Messaging API (RTM)
+# Optional if slack_hook_port and inbound_uri_prefix are defined, required otherwise.
+#
 rtm:
-    enable: true
+  # Use the RTM API to listen for requests, which does not require
+  # the bridge to listen on the hook port.
+  # You should leave this enabled, unless you plan to use the
+  # bridge exclusively for webhooks.
+  #
+  enable: true
+
+  # Logging level specific to RTM traffic.
+  #
+  log_level: "silent"
 
 {% if matrix_appservice_slack_database_engine == 'nedb' %}
 dbdir: "/data"