Add note about authentication

_source/_posts/2025-01-03-mqtt-broker.markdown

@@ -35,3 +35,8 @@ categories:
 * built-in (Mnesia) database
 * built-in web dashboard, including websocket client
 * built-in dynamic authentication with multiple backends
+
+Supports authenticating clients either by user or by client id. This setting can be changed on the fly without changing the database (as long as the identifiers are consistent).
+:bulb: Authenticating by user allows the same user account to be used for multiple client ids, e.g. with a user account corresponding to a person who can use different clients at different times to connect. Clients are expected to generate a unique client id on the fly.
+
+Authenticating based on client id forces you to provision new credentials for each individual client.