MQTT admin guide: missing documentation for Will Message, Retain, and Hooks

[cschanhniem]

Issue

The MQTT admin guide covers basic setup, access control, encryption, and WebSocket transport, but it does not document three important MQTT features:

1. Will Message (Last Will and Testament) — MQTT 3.1.1 §3.1.2.5. The mod_mqtt configuration in ejabberd supports will_topic, will_payload, will_qos, and will_retain options, but these are absent from the admin guide.
2. Retain — MQTT 3.1.1 §3.3.2.4 / §3.3.2.3. How retained messages behave, storage limits, and any ejabberd-specific configuration or restrictions for retained messages.
3. MQTT Hooks — The guide does not list available mqtt_connect / mqtt_disconnect hooks (see PR Add mqtt_connect and mqtt_disconnect hooks to mod_mqtt_session #4569 which recently added them to mod_mqtt_session).

Expected

Add sections covering these three topics to https://docs.ejabberd.im/admin/guide/mqtt/ so users migrating from other MQTT brokers or evaluating ejabberd's MQTT capabilities can find authoritative guidance.

References

• PR #4569: Add mqtt_connect and mqtt_disconnect hooks to mod_mqtt_session — merged, but hooks not in docs
• MQTT 3.1.1 spec: Will Message
• MQTT 3.1.1 spec: Retain