Modbus Client: update example description

Extend the example description so that it actually highlights the main
features shown in the example.

Also update the example to match the guidelines:
* Do not use Example in the name
* Update screenshot
* add Connectivity category

Task-number: QTBUG-110890
Change-Id: I2296371e93b7427db960081ac10efac8deae15af
Reviewed-by: André Hartmann <aha_1980@gmx.de>
(cherry picked from commit 20d4cba63221cb144537518e492187ff82a590fa)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 78 insertions, 7 deletions

diff --git a/examples/serialbus/modbus/client/doc/src/modbusclient.qdoc b/examples/serialbus/modbus/client/doc/src/modbusclient.qdoc
index dd209c3..85c1d31 100644
--- a/examples/serialbus/modbus/client/doc/src/modbusclient.qdoc
+++ b/examples/serialbus/modbus/client/doc/src/modbusclient.qdoc
@@ -1,19 +1,90 @@
-// Copyright (C) 2017 The Qt Company Ltd.
+// Copyright (C) 2023 The Qt Company Ltd.
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GFDL-1.3-no-invariants-only
 
 /*! \example modbus/client
-    \title Modbus Client example
+    \title Modbus Client
+    \ingroup qtserialbus-examples
+    \meta category Connectivity
 
     \brief The example implements a Modbus client application.
 
     The example acts as Modbus client sending Modbus request via serial line
-    and TCP respectively. The shown dialog allows the definition of standard
-    requests and displays incoming responses.
+    or TCP. The shown dialog allows the definition of standard requests and
+    displays incoming responses.
+
+    The example must be used in conjunction with the \l {Modbus server example}
+    or another Modbus device which is either connected via TCP or Serial Port.
+
+    Key classes used in this example:
+
+    \list
+        \li \l QModbusClient
+        \li \l QModbusDataUnit
+        \li \l QModbusReply
+    \endlist
 
     \image ../images/modbusclient.png
 
-    \include examples-run.qdocinc
+    \section1 Creating a QModbusClient
 
-    The example must be used in conjunction with the \l {Modbus server example}
-    or another Modbus device which is either connected via TCP or Serial Port.
+    An instance of \l QModbusClient is required to perform any communication.
+    Depending on the specified connection type, the example can instantiate
+    a \l QModbusRtuSerialClient (for serial communication), or a
+    \l QModbusTcpClient (for TCP-based communication).
+
+    \snippet modbus/client/mainwindow.cpp create_client_0
+
+    Once the client is created, use the
+    \l {QModbusClient::}{setConnectionParameter()} method to specify the
+    connection parameters. The parameters vary depending on the communication
+    type:
+
+    \snippet modbus/client/mainwindow.cpp create_client_1
+
+    After the client is created and all the parameters are specified, use
+    \l {QModbusClient::connectDevice()} to connect to Modbus network.
+
+    \section1 Reading Data
+
+    To read data from Modbus server, the client needs to specify a
+    server address and the parameters of objects that it wants to read:
+    \list
+        \li \l {QModbusDataUnit::}{RegisterType}
+        \li \l {QModbusDataUnit::startAddress}{start address}
+        \li \l {QModbusDataUnit::valueCount}{number of entries to read}
+    \endlist
+
+    The object parameters are represented by the \l QModbusDataUnit class:
+
+    \snippet modbus/client/mainwindow.cpp read_data_0
+
+    Once the parameters are collected, the
+    \l {QModbusClient::}{sendReadRequest()} method is used to send the actual
+    request. The method returns a \l QModbusReply which should be processed
+    in an asynchronous way, so the \l QModbusReply::finished() signal is used
+    to check when the reply is ready.
+
+    \snippet modbus/client/mainwindow.cpp read_data_1
+
+    Once the \l QModbusReply::finished() signal is received, the reply object
+    can be used to get the data or to check for read errors:
+
+    \snippet modbus/client/mainwindow.cpp read_data_2
+
+    \section1 Writing Data
+
+    To write the data to the Modbus server, the client needs to specify the
+    server address, and the parameters of objects that it wants to write.
+    As with reading the data, the \l QModbusDataUnit class is used to represent
+    the information about the data to be written. This time the data also
+    includes the desired \l {QModbusDataUnit::}{values}.
+    The \l {QModbusClient::}{sendWriteRequest()} method is used to write the
+    desired data:
+
+    \snippet modbus/client/mainwindow.cpp write_data_0
+
+    Like with reading the data, the returned \l QModbusReply object is used
+    to check for write errors.
+
+    \include examples-run.qdocinc
 */