Add docs for admin report content message

Part of https://github.com/vector-im/riot-web/issues/11992

docs/config.md

@@ -37,6 +37,10 @@ For a good example, see https://riot.im/develop/config.json.
        authentication flows
     1. `authFooterLinks`: a list of links to show in the authentication page footer:
       `[{"text": "Link text", "url": "https://link.target"}, {"text": "Other link", ...}]`
+1. `reportEvent`: Configures the dialog for reporting content to the homeserver
+   admin.
+    1. `adminMessageMD`: An extra message to show on the reporting dialog to
+       mention homeserver-specific policies. Accepts Markdown.
 1. `integrations_ui_url`: URL to the web interface for the integrations server. The integrations
    server is not Riot and normally not your homeserver either. The integration server settings
    may be left blank to disable integrations.