doc: zbus: document ZBUS_CHAN_DEFINE_WITH_ID

Add documentation for the `ZBUS_CHAN_DEFINE_WITH_ID` feature.

Signed-off-by: Jordan Yates <[email protected]>

Author: JordanYates

doc/services/zbus/index.rst

@@ -636,6 +636,34 @@ the defined channels and observers.
     ZBUS_CHAN_DECLARE(acc_chan, version_chan);
 
 
+Unique channel identifiers
+--------------------------
+
+To simplify integrations with external entities, it is possible to assign a unique numeric identifier
+to a channel. Users can then retrieve the channel reference by using the identifier with
+:c:func:`zbus_chan_from_id`, rather than needing to obtain the reference at compile time with
+:c:macro:`ZBUS_CHAN_DECLARE`. Channels using this feature are declared with
+:c:func:`ZBUS_CHAN_DEFINE_WITH_ID`.
+
+.. code-block:: c
+
+    ZBUS_CHAN_DEFINE_WITH_ID(control_chan,    /* Name */
+        0x12345678,              /* Unique channel identifier */
+        struct control_msg,      /* Message type */
+        control_validator,       /* Validator */
+        &message_count,          /* User data */
+        ZBUS_OBSERVERS_EMPTY,    /* observers */
+        ZBUS_MSG_INIT(.move = 0) /* Initial value */
+    );
+
+    static void channel_retrieve(void)
+    {
+        const struct zbus_channel *chan = zbus_chan_from_id(0x12345678);
+
+        ...
+    }
+
+
 Iterating over channels and observers
 =====================================