Doc: Add alt text for \image tags for qtmqtt

Alternate text (alt text) improves the documentation experience for
screen readers and for other accessibility tools.

Task-number: QTBUG-135098
Change-Id: Ia1793ff3e0bacd8cfdad2e8c04725a5093ed30aa
Reviewed-by: David Boddie <david.boddie@qt.io>
(cherry picked from commit 34656ac5ba)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

examples/mqtt/doc/quickpublication.qdoc

@@ -10,6 +10,8 @@
     topics.
 
     \image quickpublication.png
+           {MQTT publishing interface with Hostname, Port, Topic, Message, QoS,
+           and Retain fields}
 
     \e {Qt Quick Publication} demonstrates how to register QMqttClient as a QML type and
     use it in a Qt Quick application.

examples/mqtt/doc/quicksubscription.qdoc

@@ -10,6 +10,8 @@
     topics.
 
     \image quicksubscription.png
+           {MQTT subscription interface with Hostname, Port, Topic fields, Connect
+           and Subscribe buttons}
 
     \e {Qt Quick Subscription} demonstrates how to register QMqttClient as a QML type and
     use it in a Qt Quick application.

examples/mqtt/doc/simpleclient.qdoc

@@ -9,6 +9,8 @@
     \brief Creating a minimalistic telemetry application.
 
     \image simpleclient.png
+           {MQTT client with Host, Port, Protocol fields, WebSockets and Secure
+           options, Topic and Message inputs, and log showing state changes}
 
     \e {Simple MQTT Client} demonstrates how to create a minimalistic client
     application.

examples/mqtt/doc/subscriptions.qdoc

@@ -9,6 +9,8 @@
     \brief Creating a application that communicates with an MQTT broker.
 
     \image subscriptions.png
+           {Main window with connection and publishing controls, and two
+           subscription windows showing received messages on qtmqtt/topic1}
 
     \e {MQTT Subscriptions} shows how to create an application that communicates
     with an MQTT broker. A new dialog opens for each subscription, where you can

src/mqtt/doc/src/overview.qdoc

@@ -21,7 +21,8 @@
     broker. The other clients can subscribe to notifications on particular
     topics sent by the client.
 
-    \image mqtt.png
+    \image mqtt.png {Broker distributing messages from Client 1 to Client 2
+           (Topic A) and Client 3 (Topics A and B)}
 
     For example, if \e {Client 2} subscribes to messages on \e {Topic A}, it
     receives a notification when \e {Client 1} sends a message on that topic.